libstdc++: Fix std::jthread assertion and re-enable skipped test

libstdc++-v3/ChangeLog:

	* include/std/thread (jthread::_S_create): Fix static assert
	message.
	* testsuite/30_threads/jthread/95989.cc: Re-enable test.
	* testsuite/30_threads/jthread/jthread.cc: Do not require
	pthread effective target.
	* testsuite/30_threads/jthread/2.cc: Moved to...
	* testsuite/30_threads/jthread/version.cc: ...here.