Allow GHC library to be reinstalled

While the GHC library currently has a .cabal file, the information that it captures is not sufficient to build the package as a "normal" Haskell package. Consequently, ghc must be built using the GHC build system and the rest of the ecosystem (e.g. cabal-install) considers it to be non-reinstallable. This has a few unfortunate implications:

• users of the GHC API must use precisely the dependency packages that ghc links against
• GHC binary distributions must ship the ghc library in every conceivable way that the user might need

However there is no good reason why ghc can't be made to be reinstallable and shipped as a normal Hackage package (with some restrictions). Specifically, this involves:

• moving all of the ad-hoc compiler flags encoded in the build system into the .cabal file
• moving any code generation logic from the build system into a custom Setup.hs