Fix synchronization issues in <stop_token>. Replace shared_ptr with
_Stop_state_ref and a reference count embedded in the shared state.
Replace std::mutex with spinlock using one bit of a std::atomic<> that
also tracks whether a stop request has been made and how many
stop_source objects share ownership of the state.

	PR libstdc++/92895
	* include/std/stop_token (stop_token::stop_possible()): Call new
	_M_stop_possible() function.
	(stop_token::stop_requested()): Do not use stop_possible().
	(stop_token::binary_semaphore): New class, as temporary stand-in for
	std::binary_semaphore.
	(stop_token::_Stop_cb::_M_callback): Add noexcept to type.
	(stop_token::_Stop_cb::_M_destroyed, stop_token::_Stop_cb::_M_done):
	New data members for symchronization with stop_callback destruction.
	(stop_token::_Stop_cb::_Stop_cb): Make non-template.
	(stop_token::_Stop_cb::_M_linked, stop_token::_Stop_cb::_S_execute):
	Remove.
	(stop_token::_Stop_cb::_M_run): New member function.
	(stop_token::_Stop_state::_M_stopped, stop_token::_Stop_state::_M_mtx):
	Remove.
	(stop_token::_Stop_state::_M_owners): New data member to track
	reference count for ownership.
	(stop_token::_Stop_state::_M_value): New data member combining a
	spinlock, the stop requested flag, and the reference count for
	associated stop_source objects.
	(stop_token::_Stop_state::_M_requester): New data member for
	synchronization with stop_callback destruction.
	(stop_token::_Stop_state::_M_stop_possible()): New member function.
	(stop_token::_Stop_state::_M_stop_requested()): Inspect relevant bit
	of _M_value.
	(stop_token::_Stop_state::_M_add_owner)
	(stop_token::_Stop_state::_M_release_ownership)
	(stop_token::_Stop_state::_M_add_ssrc)
	(stop_token::_Stop_state::_M_sub_ssrc): New member functions for
	updating reference counts.
	(stop_token::_Stop_state::_M_lock, stop_token::_Stop_state::_M_unlock)
	(stop_token::_Stop_state::_M_lock, stop_token::_Stop_state::_M_unlock)
	(stop_token::_Stop_state::_M_try_lock)
	(stop_token::_Stop_state::_M_try_lock_and_stop)
	(stop_token::_Stop_state::_M_do_try_lock): New member functions for
	managing spinlock.
	(stop_token::_Stop_state::_M_request_stop): Use atomic operations to
	read and update state. Release lock while running callbacks. Use new
	data members to synchronize with callback destruction.
	(stop_token::_Stop_state::_M_remove_callback): Likewise.
	(stop_token::_Stop_state::_M_register_callback): Use atomic operations
	to read and update state.
	(stop_token::_Stop_state_ref): Handle type to manage _Stop_state,
	replacing shared_ptr.
	(stop_source::stop_source(const stop_source&)): Update reference count.
	(stop_source::operator=(const stop_source&)): Likewise.
	(stop_source::~stop_source()): Likewise.
	(stop_source::stop_source(stop_source&&)): Define as defaulted.
	(stop_source::operator=(stop_source&&)): Establish postcondition on
	parameter.
	(stop_callback): Enforce preconditions on template parameter. Replace
	base class with data member of new _Cb_impl type.
	(stop_callback::stop_callback(const stop_token&, Cb&&))
	(stop_callback::stop_callback(stop_token&&, Cb&&)): Fix TOCTTOU race.
	(stop_callback::_Cb_impl): New type wrapping _Callback member and
	defining the _S_execute member function.
	* testsuite/30_threads/stop_token/stop_callback/deadlock-mt.cc: New
	test.
	* testsuite/30_threads/stop_token/stop_callback/deadlock.cc: New test.
	* testsuite/30_threads/stop_token/stop_callback/destroy.cc: New test.
	* testsuite/30_threads/stop_token/stop_callback/destructible_neg.cc:
	New test.
	* testsuite/30_threads/stop_token/stop_callback/invocable_neg.cc: New
	test.
	* testsuite/30_threads/stop_token/stop_callback/invoke.cc: New test.
	* testsuite/30_threads/stop_token/stop_source/assign.cc: New test.
	* testsuite/30_threads/stop_token/stop_token/stop_possible.cc: New
	test.