libstdc++: Make Networking TS work without gthreads [PR 89760]

libstdc++: Make Networking TS work without gthreads [PR 89760]

Make the experimental Networking TS code work without std::mutex and
std::condition_variable.

libstdc++-v3/ChangeLog:

	PR libstdc++/89760
	* include/experimental/executor [!_GLIBCXX_HAS_GTHREADS]:
	(execution_context::mutex_type): Define dummy mutex type.
	(system_context): Use execution_context::mutex_type.
	(system_context) [!_GLIBCXX_HAS_GTHREADS]: Define dummy
	thread and condition variable types.
	[!_GLIBCXX_HAS_GTHREADS] (system_context::_M_run()): Do not
	define.
	(system_context::_M_post) [!_GLIBCXX_HAS_GTHREADS]: Throw
	an exception when threads aren't available.
	(strand::running_in_this_thread()): Defer to _M_state.
	(strand::_State::running_in_this_thread()): New function.
	(use_future_t): Do not depend on _GLIBCXX_USE_C99_STDINT_TR1.
	* include/experimental/io_context (io_context): Use the
	execution_context::mutex_type alias. Replace stack of thread
	IDs with counter.
	* testsuite/experimental/net/execution_context/use_service.cc:
	Enable test for non-pthread targets.