[RFC] Compatibility with GHC 7.7

Here are a few changes allowing things to build on GHC 7.7. Note that `gmp-extras.cmm` is likely broken for older GHCs at this point as we now use new-style 7.7. Also `testBitIntegerzh` is now provided by GHC. Given the number of differences between the old and new C-- code, I'm not really sure how to support both simultaneously. Thoughts?