Generate primops compatible with GHC 7.8

Add condtional compilation that enerates code compatible with both
the old comparison primops (GHC 7.6 and below) and the new ones
(GHC 7.8 and above). This changes are required to bootstrap GHC
7.8 with GHC 7.6.
Merge pull request #29 from jstolarek/ghc-78-compatibility

Generate primops compatible with GHC 7.8