Setup CI builders on OpenBSD

So far we have been fixing breakages on OpenBSD mostly reactively. An OpenBSD VM is not particularly hard to keep, here's one used by syzbot. pkg_add ghc provides the official 8.10.3 package for bootstrap.

Some known deficiencies may pose initial problems:

• hadrian build is broken on OpenBSD
• a few patches aren't integrated
• some tests don't currently pass

@angerman I could probably tackle this during the summer if @bgamari provided a bit of guidance.