patch applied (ghc-6.6/ghc): MERGED: THREADED_RTS: use cas() when claiming thunks

[Ian Lynagh]

Fri Mar 23 10:49:51 PDT 2007  Ian Lynagh <igloo at earth.li>
  * MERGED: THREADED_RTS: use cas() when claiming thunks
  Simon Marlow <simonmar at microsoft.com>**20070306142732
   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/GC.c -1 +20