Fix exec.c handling in Makefile

Fix exec.c handling in Makefile

exec.c ws handled specially in COMMON_OBS, but there doesn't seem to
be a reason for this.  This changes the Makefile to treat exec.c as an
ordinary source file.

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

	* Makefile.in (ALLDEPFILES): Remove exec.c.
	(COMMON_OBS): Remove exec.o.
	(COMMON_SFILES): Add exec.c.