libstdc++: Adjust effective-target requirements for <barrier> and <semaphore>

libstdc++: Adjust effective-target requirements for <barrier> and <semaphore>

These features depend on __cpp_lib_atomic_wait which is not available
for freestanding, and is available when either gthreads is supported, or
the target is linux (for futex support).

libstdc++-v3/ChangeLog:

	* testsuite/30_threads/barrier/1.cc: Require hosted. Only
	require gthreads for non-linux targets.
	* testsuite/30_threads/barrier/2.cc: Likewise.
	* testsuite/30_threads/semaphore/1.cc: Likewise.
	* testsuite/30_threads/semaphore/2.cc: Likewise.
	* testsuite/30_threads/semaphore/cons.cc: Likewise.
	* testsuite/30_threads/semaphore/least_max_value_neg.cc:
	Likewise.
	* testsuite/30_threads/semaphore/try_acquire.cc: Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>