[libatomic] Add RTEMS support

gcc/

	* config/rtems.h (LIB_SPEC): Add -latomic.

libatomic/

	* configure.tgt (configure_tgt_pre_target_cpu_XCFLAGS): New variable.
	(*-*-rtems*): New supported target.
	* config/rtems/host-config.h: New file.
	* config/rtems/lock.c: Likewise.

From-SVN: r235466