Add support for GHCJS

This provides a (AFAICT) fully working GHCJS package set.

@peti Can I get your feedback on how to best merge this in? I'm sure there are some things to consider, like if/how we should specify that a given package supports GHCJS (or more generically, any particular compiler).
I mean, is it important that `haddock` be fixed in that way across the entire `packages_ghc784` package set, or is it safe to localize that change to just `ghcjs`?