gdb/testsuite: add macros test for source files compiled in various ways

gdb/testsuite: add macros test for source files compiled in various ways

Using different ways of passing source file paths to compilers results n
different file and directory paths in the line header.  For example:

  - gcc foo.c
  - gcc ./foo.c
  - gcc ../cwd/foo.c
  - gcc $PWD/foo.c

Because of this, GDB sometimes failed to look up macros.  The previous
patch fixed that as much as possible.  This patch adds the corresponding
tests.

Add both a DWARF assembler-based test and a regular test.  The DWARF
assembled-based one tests some hard-coded debug info based on what I
have observed some specific versions of gcc and clang generate.  We want
to make sure that GDB keeps handling all these cases correctly, even if
it's not always clear whether they are really valid DWARF.  Also, they
will be tested no matter what the current target compiler is for a given
test run.

The regular test is compiled using the target compiler, so it may help
find bugs when testing against some other toolchains than what was used
to generate the DWARF assembler-based test.

For the DWARF assembler-based test, add to testsuite/lib/dwarf.exp the
necessary code to generate a DWARF5 .debug_macro section.  The design of
the new procs is based on what was done for rnglists and loclists.

To test against a specific compiler one can use this command, for
example:

    $ make check TESTS="gdb.base/macro-source-path.exp" RUNTESTFLAGS="CC_FOR_TARGET=clang --target_board unix/gdb:debug_flags=-gdwarf-5"

Change-Id: Iab8da498e57d10cc2a3d09ea136685d9278cfcf6