libstdc++: Throw instead of segfaulting in std::thread constructor [PR 67791]

libstdc++: Throw instead of segfaulting in std::thread constructor [PR 67791]

This turns a mysterious segfault into an exception with a more useful
message. If the exception isn't caught, the user sees this instead of
just a segfault:

terminate called after throwing an instance of 'std::system_error'
  what():  Enable multithreading to use std::thread: Operation not permitted
Aborted (core dumped)

libstdc++-v3/ChangeLog:

	PR libstdc++/67791
	* src/c++11/thread.cc (thread::_M_start_thread(_State_ptr, void (*)())):
	Check that gthreads is available before calling __gthread_create.