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).
The change I proposed didn't force a rebuild of `ghcjs`, so I figure it must be a safe change to make. This also caries the benefit of making `ghcjs` build "out of the box" in the `packages_ghc784` package set. So, I'm going to make that change, and I can change it back if you have an objection to that.