libstdc++: Fix <stop_token> to compile without gthreads

libstdc++-v3/ChangeLog:

	* include/std/stop_token: Check _GLIBCXX_HAS_GTHREADS using
	#ifdef instead of #if.
	(stop_token::_S_yield()): Check _GLIBCXX_HAS_GTHREADS before
	using __gthread_yield.