libstdc++: Assigning to a joinable std::jthread calls std::terminate

libstdc++: Assigning to a joinable std::jthread calls std::terminate

Move assigning to a std::jthread that represents a thread of execution
needs to send a stop request and join that running thread. Otherwise the
std::thread data member will terminate in its assignment operator.

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

	* include/std/thread (jthread::operator=(jthread&&)): Transfer
	any existing state to a temporary that will request a stop and
	then join.
	* testsuite/30_threads/jthread/jthread.cc: Test move assignment.