FreeBSD/arm64 becoming Tier 1 in FreeBSD 13

Maybe not massively relevant right now, but with Arm making its way into both servers and desktops, this is some good future-proofing for FreeBSD.

FreeBSD will promote arm64 to a Tier 1 architecture in FreeBSD 13. This means we will provide release images, binary packages, and security and errata updates. While we anticipate there will be minor issues with this first release, we believe the port is mature enough that they can be resolved during the life of FreeBSD 13.