Add MO_AddIntC, MO_SubIntC MachOps and implement in X86 backend

Summary:
These MachOps are used by addIntC# and subIntC#, which in turn are
used in integer-gmp when adding or subtracting small Integers. The
following benchmark shows a ~6% speedup after this commit on x86_64
(building GHC with BuildFlavour=perf).

    {-# LANGUAGE MagicHash #-}

    import GHC.Exts
    import Criterion.Main

    count :: Int -> Integer
    count (I# n#) = go n# 0
      where go :: Int# -> Integer -> Integer
            go 0# acc = acc
            go n# acc = go (n# -# 1#) $! acc + 1

    main = defaultMain [bgroup "count"
                          [bench "100" $ whnf count 100]]

Differential Revision: https://phabricator.haskell.org/D140