FWIW, I would also prefer that we fix libstdc++ to not depend on this
misfeature and properly implement pthread_once() using _libc_once() in
the non-threaded case as well.
One advantage of fixing libstdc++ to honor __isthreaded would be that
single threaded C++ apps that linked against libthr (due to using 3rd
party libraries that can use threads but the specific app doesn't enable
that, e.g. protobuf) would not always use locks unnecessarily in all of