- Now that labels are always prefixed with '&' in .hc code, we have to
  fix some sloppiness in the RTS .cmm code.  Fortunately it's not too
  painful.

- SMP: acquire/release the storage manager lock around
  atomicModifyMutVar#.  This is a hack: atomicModifyMutVar# isn't
  atomic under SMP otherwise, but the SM lock is a large sledgehammer.
  I think I'll apply the sledgehammer to the MVar primitives too, for
  the time being.