libstdc++: Fix __thread_yield for non-gthreads targets

libstdc++: Fix __thread_yield for non-gthreads targets

The __gthread_yield() function is only defined for gthreads targets, so
check _GLIBCXX_HAS_GTHREADS before using it.

Also reorder __thread_relax and __thread_yield so that the former can
use the latter instead of repeating the same preprocessor checks.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h (__thread_yield()): Check
	_GLIBCXX_HAS_GTHREADS before using __gthread_yield.
	(__thread_relax()): Use __thread_yield() instead of repeating
	the preprocessor checks for __gthread_yield.