The new inline definition of std::thread::hardware_concurrency() for
non-gthreads targets is missing the noexcept-specifier that is on the
declaration.

libstdc++-v3/ChangeLog:

	PR libstdc++/100298
	* include/bits/std_thread.h (thread::hardware_concurrency): Add
	missing noexcept to inline definition for non-gthreads targets.

Tested x86_64-linux. Committed to trunk, about to backport to gcc-11.