testsuite: Add dg-require-atomic-cmpxchg-word

testsuite: Add dg-require-atomic-cmpxchg-word

Some targets (armv6-m) support inline atomic load and store,
i.e. dg-require-thread-fence matches, but not atomic operations like
compare and exchange.

This directive can be used to replace uses of dg-require-thread-fence
where an atomic operation is actually used.

	* testsuite/lib/dg-options.exp (dg-require-atomic-cmpxchg-word):
	New proc.
	* testsuite/lib/libstdc++.exp (check_v3_target_atomic_cmpxchg_word):
	Ditto.