New primop (mingw only),

  asyncDoProc# :: Addr# -> Addr# -> State# RealWorld-> (# State# RealWorld, Int#, Int# #)

which lets a Haskell thread hand off a pointer to external code (1st arg) for
asynchronous execution by the RTS worker thread pool. Second arg is data passed
in to the asynchronous routine. The routine is _not_ permitted to re-enter
the RTS as part of its execution.