Negated GHC flags don't cancel out in package hash

[georgefst]

As a concrete example, I enable ghc-options: -haddock in my global config, and pass --ghc-options=-no-haddock when cross-compiling. As a result, much of my store has cabal-hash.txts with ghc-options: -haddock -no-haddock, which is the same in principle as passing no GHC flags at all.

This has the potential to cause some cache misses.