Use a stamp file for init.c

Use a stamp file for init.c

This introduces a stamp file for init.c.  This prevents constant
rebuilds of init.o, by arranging for init.c to only be modified when
its contents change.  (FWIW this is a standard idiom in use by
Automake and by gdb itself for many years.)

gdb/ChangeLog
2018-07-09  Tom Tromey  <tom@tromey.com>

	* Makefile.in (init.c): Depend on stamp-init.
	(stamp-init): New rule, from init.c rule.
	(clean mostlyclean): Remove stamp-init.