patch applied (ghc): add MO_WriteBarrier to CallishMachOps

[Simon Marlow]

Thu Jun 29 04:58:37 PDT 2006  Simon Marlow <simonmar at microsoft.com>
  * add MO_WriteBarrier to CallishMachOps
  This will let us express write barriers in C--

    M ./compiler/cmm/MachOp.hs +1
    M ./compiler/cmm/PprC.hs +1
    M ./compiler/nativeGen/MachCodeGen.hs +8