Use isTrue# for GHC >= 7.7 compatibility
gmp-extras.cmm: Compatibility with GHC >= 7.7

Using ghc commit a7c0387d20c1c9994d1100b14fbb8fb4e28a259e as a guide for
porting to new-style C--. This likely breaks compatibility with < 7.7.

TODO: Fix compatibility
Build-dep bumps