Distinguish between the scheduling of a new thread from within
the RTS (e.g., via forkIO, running finalizers etc) and scheduling
of a thread that's created via the RtsAPI -- the latter
now uses scheduleExtThread(), the rest scheduleThread().

Why the distinction? Because the former will in threaded builds create
a worker OS thread, while the latter won't. (There's an added
wrinkle -- main() will also use scheduleThread()).