patch applied (ghc): THREADED_RTS: use cas() when claiming thunks

[Simon Marlow]

Tue Mar  6 06:27:32 PST 2007  Simon Marlow <simonmar at microsoft.com>
  * THREADED_RTS: use cas() when claiming thunks
  I guess I forgot to do this the first time around; the upshot is that
  there could be some uncaught duplication of work on a multiprocessor
  (but unlikely).

    M ./rts/ThreadPaused.c -1 +21