patch applied (ghc): copy_tag_nolock(): fix write ordering and add a write_barrier()

[Simon Marlow]

Tue Mar 16 07:31:03 PDT 2010  Simon Marlow <marlowsd at gmail.com>
  * copy_tag_nolock(): fix write ordering and add a write_barrier()
  
  Fixes a rare crash in the parallel GC.
  
  If we copy a closure non-atomically during GC, as we do for all
  immutable values, then before writing the forwarding pointer we better
  make sure that the closure itself is visible to other threads that
  might follow the forwarding pointer.  I imagine this doesn't happen
  very often, but I just found one case of it: in scavenge_stack, the
  RET_FUN case, after evacuating ret_fun->fun we then follow it and look
  up the info pointer.

    M ./rts/sm/Evac.c -2 +6