Get rid of multiple-result MachOps (MO_NatS_AddC, MO_NatS_SubC,
MO_NatS_MulC) which implement {add,sub,mul}IntC#.  Supporting gunk
in the NCG disappears as a result.

Instead:

* {add,sub}IntC# are translated out during abstract C simplification,
  turning into the xor-xor-invert-and-shift sequence previously defined
  in PrimOps.h.

* mulIntC# is more difficult to get rid of portably.  Instead we have
  a new single-result PrimOp, mulIntMayOflo, with corresponding MachOp
  MO_NatS_MulMayOflo.  This tells you whether a W x W -> W signed
  multiply might overflow, where W is the word size.  When W=32, is
  implemented by computing a 2W-long result.  When W=64, we use the
  previous approximation.

PrelNum.lhs' implementation of timesInteger changes slightly, to use
the new PrimOp.