Don't include selftests objects in build when unit tests are disabled

Don't include selftests objects in build when unit tests are disabled

While working on the preceding selftests patches, I noticed that some
selftests-specific files are included in the build even when selftests
are disabled, namely disasm-selftest.c and gdbarch-selftests.c.  These
files are entirely #if'ed out when building with selftests disabled.

This is not a huge problem, but I think it would make more sense if
these files were simply not built.

With this patch, I propose to put all the selftests-specific source
files into a SELFTESTS_SRCS Makefile variable (even selftest-arch.c,
which is currently added by the configure script).

gdb/ChangeLog:

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Rename to...
	(SELFTESTS_SRCS): ... this.  Add disasm-selftests.c,
	gdbarch-selfselftests.c and selftest-arch.c.
	(SUBDIR_UNITTESTS_OBS): Rename to...
	(SELFTESTS_OBS): ... this.
	(COMMON_SFILES): Remove disasm-selftests.c and
	gdbarch-selftests.c.
	* configure.ac: Don't add selftest-arch.{c,o} to
	CONFIG_{SRCS,OBS}.
	* disasm-selftests.c, gdbarch-selftests.c: Remove GDB_SELF_TEST
	preprocessor conditions.