arch.7: update applicable FreeBSD versions to 12.0 and later

Information in this document is unchanged between 11.x and 12.x, but
this is intended to be a quick reference for supported architectures.
Also bump .Dd to cover recent changes including MIPS deprecation.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 9c983ad791c3ca8213cae949f9ad381d1309ad8e)
(cherry picked from commit c654a0839f87336cfc5d747702cb55e2dd616c7e)