lang/ghc810: Add the port, slave to lang/ghc.

The need to have different GHC versions installed on the same system became
more common in the Haskell ecosystem. The devel/stack port solves this problem
by installing its own distributions into the user's HOME dir. Cabal doesn't do
that, but uses system-wide GHC installations. This slave port is intended to
help for such use cases.

This port can also be used as a fallback during major lang/ghc upgrades. At
the moment such upgrades require a lot of work to deal with Haskell packages
fallout. By switching failing ports to older GHC we can deliver lang/ghc
updates faster.