Makefile: Replace explicit subdir rules with pattern rules

Makefile: Replace explicit subdir rules with pattern rules

When adding a .c file in subdirectory (e.g. mi/), the current practice
is to add an explicit rule, such as:

  mi-cmd-break.o: $(srcdir)/mi/mi-cmd-break.c
          $(COMPILE) $(srcdir)/mi/mi-cmd-break.c
          $(POSTCOMPILE)

I find it a bit verbose and cumbersome.  Since we now require GNU make,
we can change those rules with pattern rules, one for each subdirectory.
For example, the following rule works for all files under mi:

  %.o: $(srcdir)/mi/%.c
          $(COMPILE) $<
          $(POSTCOMPILE)

Those pattern rules assume that the source and target files have the
same stem (foo.c and foo.o).  In one case, common-agent.o is generated
from common/agent.c, to avoid a conflict with the agent.o in gdb/.  In
this case, I kept the explicit rule, which takes precedence over the
pattern rule.  We could also rename common/agent.c to
common/common-agent.c to get rid of the special case and still avoid the
clash, as it is done with common/common-regcache.c, for example.

This strategy was the least intrusive I found, as it only requires
changing the rules, not the target names.

I also considered two other solutions, which I did not like because I
would have had to change target names a bit everywhere.

  - Replicate the source directory structure in the build directory,
    which would generate common/agent.o from common/agent.c.  However,
    something was not right with the dependency tracking (the .deps
    directory).  It's probably not hard to fix, but I did not
    investigate further.
  - Name the object files after the directory they are in, so that
    common/agent.c would generate common_agent.c.

GDBserver can benefit from the same treatment, but I'll do it in another
patch.

Built-tested with --enable-targets=all.

New in v2:

  - Regroup pattern rules for .c -> .o compilation in a single place.
  - Add comment about common-agent.o.

gdb/ChangeLog:

	(PYTHON_CFLAGS): Move up.
	(%.o: $(srcdir)/arch/%.c): New rule.
	(%.o: $(srcdir)/cli/%.c): New rule.
	(%.o: $(srcdir)/common/%.c): New rule.
	(%.o: $(srcdir)/compile/%.c): New rule.
	(%.o: $(srcdir)/gdbtk/generic/%.c): New rule.
	(%.o: $(srcdir)/guile/%.c): New rule.
	(%.o: $(srcdir)/mi/%.c): New rule.
	(%.o: $(srcdir)/nat/%.c): New rule.
	(%.o: $(srcdir)/python/%.c): New rule.
	(%.o: $(srcdir)/target/%.c): New rule.
	(%.o: $(srcdir)/tui/%.c): New rule.
	(cli-cmds.o): Remove.
	(cli-decode.o): Likewise.
	(cli-dump.o): Likewise.
	(cli-interp.o): Likewise.
	(cli-logging.o): Likewise.
	(cli-script.o): Likewise.
	(cli-setshow.o): Likewise.
	(cli-utils.o): Likewise.
	(compile.o): Likewise.
	(compile-c-types.o): Likewise.
	(compile-c-symbols.o): Likewise.
	(compile-object-load.o): Likewise.
	(compile-object-run.o): Likewise.
	(compile-loc2c.o): Likewise.
	(compile-c-support.o): Likewise.
	(gdbtk.o): Likewise.
	(gdbtk-bp.o): Likewise.
	(gdbtk-cmds.o): Likewise.
	(gdbtk-hooks.o): Likewise.
	(gdbtk-interp.o): Likewise.
	(gdbtk-main.o): Likewise.
	(gdbtk-register.o): Likewise.
	(gdbtk-stack.o): Likewise.
	(gdbtk-varobj.o): Likewise.
	(gdbtk-wrapper.o): Likewise.
	(mi-cmd-break.o): Likewise.
	(mi-cmd-catch.o): Likewise.
	(mi-cmd-disas.o): Likewise.
	(mi-cmd-env.o): Likewise.
	(mi-cmd-file.o): Likewise.
	(mi-cmd-info.o): Likewise.
	(mi-cmds.o): Likewise.
	(mi-cmd-stack.o): Likewise.
	(mi-cmd-target.o): Likewise.
	(mi-cmd-var.o): Likewise.
	(mi-console.o): Likewise.
	(mi-getopt.o): Likewise.
	(mi-interp.o): Likewise.
	(mi-main.o): Likewise.
	(mi-out.o): Likewise.
	(mi-parse.o): Likewise.
	(mi-symbol-cmds.o): Likewise.
	(mi-common.o): Likewise.
	(signals.o): Likewise.
	(common-utils.o): Likewise.
	(gdb_vecs.o): Likewise.
	(xml-utils.o): Likewise.
	(ptid.o): Likewise.
	(buffer.o): Likewise.
	(filestuff.o): Likewise.
	(format.o): Likewise.
	(vec.o): Likewise.
	(print-utils.o): Likewise.
	(rsp-low.o): Likewise.
	(errors.o): Likewise.
	(common-debug.o): Likewise.
	(cleanups.o): Likewise.
	(common-exceptions.o
	(posix-strerror.o): Likewise.
	(mingw-strerror.o): Likewise.
	(btrace-common.o): Likewise.
	(fileio.o): Likewise.
	(common-regcache.o): Likewise.
	(signals-state-save-restore.o): Likewise.
	(new-op.o): Likewise.
	(waitstatus.o): Likewise.
	(arm.o): Likewise.
	(arm-linux.o): Likewise.
	(arm-get-next-pcs.o): Likewise.
	(x86-dregs.o): Likewise.
	(linux-btrace.o): Likewise.
	(linux-osdata.o): Likewise.
	(linux-procfs.o): Likewise.
	(linux-ptrace.o): Likewise.
	(linux-waitpid.o): Likewise.
	(mips-linux-watch.o): Likewise.
	(ppc-linux.o): Likewise.
	(linux-personality.o): Likewise.
	(x86-linux.o): Likewise.
	(x86-linux-dregs.o): Likewise.
	(amd64-linux-siginfo.o): Likewise.
	(linux-namespaces.o): Likewise.
	(aarch64-linux-hw-point.o): Likewise.
	(aarch64-linux.o): Likewise.
	(aarch64-insn.o): Likewise.
	(tui.o): Likewise.
	(tui-command.o): Likewise.
	(tui-data.o): Likewise.
	(tui-disasm.o): Likewise.
	(tui-file.o): Likewise.
	(tui-hooks.o): Likewise.
	(tui-interp.o): Likewise.
	(tui-io.o): Likewise.
	(tui-layout.o): Likewise.
	(tui-out.o): Likewise.
	(tui-regs.o): Likewise.
	(tui-source.o): Likewise.
	(tui-stack.o): Likewise.
	(tui-win.o): Likewise.
	(tui-windata.o): Likewise.
	(tui-wingeneral.o): Likewise.
	(tui-winsource.o): Likewise.
	(guile.o): Likewise.
	(scm-arch.o): Likewise.
	(scm-auto-load.o): Likewise.
	(scm-block.o): Likewise.
	(scm-breakpoint.o): Likewise.
	(scm-cmd.o): Likewise.
	(scm-disasm.o): Likewise.
	(scm-exception.o): Likewise.
	(scm-frame.o): Likewise.
	(scm-gsmob.o): Likewise.
	(scm-iterator.o): Likewise.
	(scm-lazy-string.o): Likewise.
	(scm-math.o): Likewise.
	(scm-objfile.o): Likewise.
	(scm-param.o): Likewise.
	(scm-ports.o): Likewise.
	(scm-pretty-print.o): Likewise.
	(scm-progspace.o): Likewise.
	(scm-safe-call.o): Likewise.
	(scm-string.o): Likewise.
	(scm-symbol.o): Likewise.
	(scm-symtab.o): Likewise.
	(scm-type.o): Likewise.
	(scm-utils.o): Likewise.
	(scm-value.o): Likewise.
	(python.o): Likewise.
	(py-arch.o): Likewise.
	(py-auto-load.o): Likewise.
	(py-block.o): Likewise.
	(py-bpevent.o): Likewise.
	(py-breakpoint.o): Likewise.
	(py-cmd.o): Likewise.
	(py-continueevent.o): Likewise.
	(py-xmethods.o): Likewise.
	(py-event.o): Likewise.
	(py-evtregistry.o): Likewise.
	(py-evts.o): Likewise.
	(py-exitedevent.o): Likewise.
	(py-finishbreakpoint.o): Likewise.
	(py-frame.o): Likewise.
	(py-framefilter.o): Likewise.
	(py-function.o): Likewise.
	(py-gdb-readline.o): Likewise.
	(py-inferior.o): Likewise.
	(py-infevents.o): Likewise.
	(py-infthread.o): Likewise.
	(py-lazy-string.o): Likewise.
	(py-linetable.o): Likewise.
	(py-newobjfileevent.o): Likewise.
	(py-objfile.o): Likewise.
	(py-param.o): Likewise.
	(py-prettyprint.o): Likewise.
	(py-progspace.o): Likewise.
	(py-signalevent.o): Likewise.
	(py-stopevent.o): Likewise.
	(py-symbol.o): Likewise.
	(py-symtab.o): Likewise.
	(py-threadevent.o): Likewise.
	(py-type.o): Likewise.
	(py-unwind.o): Likewise.
	(py-utils.o): Likewise.
	(py-value.o): Likewise.
	(py-varobj.o): Likewise.