Define `Ord` on `GhcFlavor`

[shayne-fletcher]

See issue #264 (comment). Consider defining an ordering on GhcLibGenOpts.GhcFlavor and use it to simplify code like ghcFlavor `elem` [ GhcMaster, Ghc8101, Ghc8102, Ghc8103, Ghc901 ] ] to ghcFlavor >= Ghc8101.