Add missing `{-# LANGUAGE #-}` pragmas

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Modernize `ghc-prim.cabal` to `cabal-version>=1.10`

This also removes the use of `makefileHook` in `Setup.hs` which was
dropped from the `Cabal` API

Note: This cleanups make `ghc-prim` almost a proper Cabal package;
`cabal sdist` still needs to be tweaked to change `build-type` to
`Simple`, to make the resulting ghc-prim source distribution package
self-contained.