Add comprehensive matrix of GHC bindists

[iand675]

General summary/comments

I attempted to do a stack setup on MacOS of the integersimple variant of GHC 8.0.2. However, that's apparently not currently supported. It's my understanding that interested users are currently expected to provide these bindist variations, but I didn't have much luck finding a list that shows what's currently available.

Ideally, I think it'd be great to have a table somewhere easily discoverable (perhaps even accessible via the CLI itself) with a format sort of like:

GHC Version	Host architecture	Host platform	Target architecture	Target platform	GHC variant (standard / integersimple)

[borsboom]

This could be auto-generated from https://github.com/fpco/stackage-content/blob/master/stack/stack-setup-2.yaml.