PR libstdc++/57060
	* include/std/thread (operator==(thread::id, thread::id)): Do not use
	__gthread_equal.
	(operator<(thread::id, thread::id)): Add comment.
	(this_thread::get_id()): Do not use __gthread_self for single-threaded
	programs using glibc.
	* testsuite/30_threads/this_thread/57060.cc: New.

From-SVN: r231289