libstdc++: Define (and use) _GLIBCXX_HAVE_ATOMIC_WAIT

libstdc++: Define (and use) _GLIBCXX_HAVE_ATOMIC_WAIT

In order to simplify the preprocessor checks for whether __atomic_wait
is available, this commit does:

-#if defined _GLIBCXX_HAS_GTHREADS || _GLIBCXX_HAVE_LINUX_FUTEX
+#ifdef _GLIBCXX_HAVE_ATOMIC_WAIT

The original was wrong anyway, as it should have used 'defined' to check
_GLIBCXX_HAVE_LINUX_FUTEX (for consistency with how that's used
elsewhere).

The new macro is defined in <bits/atomic_wait.h> when the file is
defines __atomic_wait and related facilities. All other code that
depends on those features can just check the one macro.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h (_GLIBCXX_HAVE_ATOMIC_WAIT):
	Define.
	* include/bits/atomic_base.h: Check _GLIBCXX_HAVE_ATOMIC_WAIT.
	* include/bits/atomic_timed_wait.h: Likewise.
	* include/bits/semaphore_base.h: Likewise.
	* include/std/atomic: Likewise.
	* include/std/latch: Likewise.
	* include/std/semaphore: Likewise.