libstdc++: Fix "bare" notifications dropped by waiters check

libstdc++: Fix "bare" notifications dropped by waiters check

For types that track whether or not there extant waiters (e.g.
semaphore) internally, the __atomic_notify_address_bare() call was
introduced to avoid the overhead of loading the atomic count of
waiters. For platforms that don't have Futex, however, there was
still a check for waiters, and seeing that there are none (because
in the bare case, the count is not incremented), the notification
is dropped. This commit addresses that case.

libstdc++-v3/ChangeLog:
	* include/bits/atomic_wait.h: Always notify waiters in the
	case of 'bare' address notification.