libstdc++: Make std::this_thread functions work without gthreads

The only function in namespace std::this_thread that actually depends on
thread support being present is this_thread::get_id(). The other
functions (yield, sleep_for and sleep_until) can be defined for targets
without gthreads.

A small change is needed in std::this_thread::sleep_for which currently
uses the __gthread_time_t typedef. Since it just calls nanosleep
directly, it should use timespec directly instead of the typedef.

Even std::this_thread::get_id() could be made to work, the only
difficulty is that it returns a value of type std::thread::id and
std::thread is only defined when gthreads support exists.

libstdc++-v3/ChangeLog:

	* include/std/thread [!_GLIBCXX_HAS_GTHREADS] (this_thread::yield)
	(this_thread::sleep_until): Define.
	[!_GLIBCXX_HAS_GTHREADS] (this_thread::sleep_for): Define. Replace
	use of __gthread_time_t typedef with timespec.
	* src/c++11/thread.cc [!_GLIBCXX_HAS_GTHREADS] (__sleep_for):
	Likewise.
	* testsuite/30_threads/this_thread/2.cc: Moved to...
	* testsuite/30_threads/this_thread/yield.cc: ...here.
	* testsuite/30_threads/this_thread/3.cc: Moved to...
	* testsuite/30_threads/this_thread/sleep_for-mt.cc: ...here.
	* testsuite/30_threads/this_thread/4.cc: Moved to...
	* testsuite/30_threads/this_thread/sleep_until-mt.cc: ...here.
	* testsuite/30_threads/this_thread/58038.cc: Add
	dg-require-sleep.
	* testsuite/30_threads/this_thread/60421.cc: Likewise.
	* testsuite/30_threads/this_thread/sleep_for.cc: New test.
	* testsuite/30_threads/this_thread/sleep_until.cc: New test.