Move target object files to target subdirectory

Move target object files to target subdirectory

Move the object files corresponding to target/*.c to the target
subdirectory in the build tree.

ChangeLog
2017-11-27  Tom Tromey  <tom@tromey.com>

	* Makefile.in (SUBDIR_TARGET_SRCS, SUBDIR_TARGET_OBS): New
	variables.
	(SFILES): Use SUBDIR_TARGET_SRCS.
	(COMMON_OBS): Use SUBDIR_TARGET_OBS.  Remove waitstatus.o.
	(CONFIG_SRC_SUBDIR): Add target.
	(%.o): Remove target rule.