libstdc++: Use helper type for checking thread ID

This encapsulates the storing and checking of the thread ID into a class
type, so that the macro _GLIBCXX_HAS_GTHREADS is only checked in one
place. The code doing the checks just calls member functions of the new
type, without caring whether that really does any work or not.

libstdc++-v3/ChangeLog:

	* include/std/stop_token (_Stop_state_t::_M_requester): Define
	new struct with members to store and check the thread ID.
	(_Stop_state_t::_M_request_stop()): Use _M_requester._M_set().
	(_Stop_state_t::_M_remove_callback(_Stop_cb*)): Use
	_M_requester._M_is_current_thread().