libstdc++: Fix __thread_relax for non-gthreads non-x86 targets

libstdc++: Fix __thread_relax for non-gthreads non-x86 targets

My recent change to the preprocessor conditions in __thread_relax() was
supposed to also change the __gthread_yield() call to __thread_yield(),
which has the right preprocessor checks. Instead I just removed the
check for _GLIBCXX_USE_SCHED_YIELD which means the __gthread_yield()
call will be ill-formed for non-gthreads targets, and targets without
sched_yield(). This fixes it properly.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h (__thread_relax()): Call
	__thread_yield() not __gthread_yield().