eclass/haskell-cabal.eclass: new function 'replace-hcflags()'

Allows masking/working-around bugs in ghc when it fails
to builds things properly on higher optimisation levels:

In our case:

- dev-haskell/misfortune - hangs indefinitely
- dev-haskell/pandoc-citeproc - eats ghc's haskell stack and crashes

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>