Make init.c depend on source files

Make init.c depend on source files

I noticed that init.c depends on the object files that go into gdb.
Because init.c actually only requires the contents of the
corresponding source files, this unnecessarily serializes the step
that builds init.c.

This patch changes gdb's Makefile to make init.c depend on the source
files.  This also simplifies the rule to build init.c.

gdb/ChangeLog
2018-12-27  Tom Tromey  <tom@tromey.com>

	* Makefile.in (INIT_FILES): Redefine.
	(stamp-init): Remove sed, tr invocations.  Use for loop.  Don't
	set LANG or LC_ALL.