Introduce convenient means of modifying compiler flavour

[bgamari]

Currently it is quite unclear whether/how one can change the build configuration of the compiler itself. For instance, building with the +debug_info, +debug_ghc, or +assertions flavour transformers is a common first step I take when faced with a new issue.

Not having these options significantly complicates the process of debugging issues only reproducible under haskell.nix.