libstdc++: Make std::jthread support pointers to member functions [PR 100612]

libstdc++: Make std::jthread support pointers to member functions [PR 100612]

This adds a non-standard extension to support initializing a
std::jthread with a pointer to a member function that expects a
stop_token to be added to the arguments. That use case is not supported
by C++20, because the stop_token would get added as the first argument,
which is where the object argument needs to be to invoke a pointer to
member function.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/100612
	* include/std/thread (__pmf_expects_stop_token): New variable
	template to detect a pointer to member function that needs a
	stop_token to be added to the arguments.
	(jthread::__S_create): Use __pmf_expects_stop_token.
	(jthread::__S_create_pmf): New function.
	* testsuite/30_threads/jthread/100612.cc: New test.