x/build: we have no builders for openbsd/arm64

[bcmills]

As of CL 174125, cmd/dist supports openbsd/arm64 (#31656), and the Go 1.13 release notes currently claim that “Go now supports OpenBSD on arm64.”

However, as far as I can tell we have no openbsd/arm64 builders, so that support does not seem to meet the requirements for a new port.

We should either get an openbsd/arm64 builder up and running and verify that it is passing, or remove the claim of support from the release notes.

CC @4a6f656c @bradfitz @ianlancetaylor @andybons @toothrot @dmitshur

[gopherbot]

Change https://golang.org/cl/192997 mentions this issue: doc/go1.13: remove announcements of arm64 support on NetBSD and OpenBSD

[gopherbot]

Change https://golang.org/cl/192998 mentions this issue: [release-branch.go1.13] doc/go1.13: remove announcements of arm64 support on NetBSD and OpenBSD

[gopherbot]

Change https://golang.org/cl/250117 mentions this issue: dashboard: add openbsd-arm64-joelsing builder (reverse)