Add FreeBSD support

Created by: gwils

I'm interested in having a crack at this.

Check out https://github.com/haskell/ghcup/blob/master/CONTRIBUTING.md#adding-a-new-distro-updating-ghc-versions- and https://github.com/haskell/ghcup/commit/6008d8a964461222ca7d712b095bb0858c1983f9

Created by: gwils

There don't seem to be FreeBSD builds up for GHC 8.6.2 or any modern cabal-install versions, so I'm going to have to tackle that before I can do this. I don't know when I'll have time to do so.

Note that it actually doesn't matter where these things are uploaded, since we are free to use any url, as long as it was built the same way as upstream builds GHC.

Created by: gwils

That's a good point I hadn't considered, but if I have to build these things and upload them somewhere anyway, it may as well be downloads.haskell.org

https://github.com/haskell/ghcup/pull/70

Preliminary support was added in #70