Fix a race condition/possible deadlock in the threaded rts:

If a callback into haskell finished before waitThread_() was called,
the signal was lost  ans waitThread_() waited indefinitely.

Solution: Don't release the sched_mutex between calls to scheduleThread_
and waitThread_.

Please note that the scheduler API function waitThread is still possibly
affected by this race condition. It's used in rts_mainEvalIO (I think that's
safe) and in finishAllThreads (this looks dangerous, but finishAllThreads is
never used).