atomic: Create and use maybe_emit_atomic_test_and_set.

atomic: Create and use maybe_emit_atomic_test_and_set.

	* optabs.c (CODE_FOR_atomic_test_and_set): Provide default.
	(maybe_emit_atomic_test_and_set): New.
	(expand_sync_lock_test_and_set): Use it.
	(expand_atomic_test_and_set): Likewise.
	* doc/extend.texi (__atomic_test_and_set): Adjust the docs to match
	the implementation; clarify implementation defined details.
	* doc/md.texi (atomic_test_and_set): Document.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@183483 138bc75d-0d04-0410-961f-82ee72b054a4