Use global architecture switch for FreeBSD ABI #1196
Alternative implementation of #1140. I'm not super happy with the way the casing works but I think this can extend to cover platforms like ARM, MIPS, SPARC, etc down the track. There is a pseudo-authoritative list of valid architecture and wordsize options here but I've only really kept the x86 parts of it. Since i386 and x86_64 both use the same architecture (x86), I've just set this as is for now.
Overall I feel like this implementation is ugly but I can't think of a nicer way to do it. This is at least more consistent with what the other package architecture mutators do, like for RPM or Deb.