Add GHC version tags

[zliu41]

There doesn't seem to be an easy way to tell which commit should be used for which GHC version. It would be helpful to add some tags for that purpose.

[bgamari]

In principle, you should be able to pass, e.g., --argstr version 8.10 to nix-shell to specify which GHC version is being built. IMHO we should try to maintain this for the last three major GHC versions.