Deorbiting armv4 and armv5 support

[Warner Losh]

Greetings,

The pmap in armv4/5 has been broken for years at this point. It basically
works, but has issues with unaligned buffers. Since these have remained
unfixed for a long time, and most of the main ARM developers have given up
trying to fix it, I think it would be best to retire the support rather
than waste people's time that seem to be working only to discover after a
lot of effort that it's busted.

Since the consensus at the FreeBSD developer's summit appeared to be 'let's
let it go'. It would remove the TARGET_ARCH arm and armeb. armv6 would
remain unaffected (though see a parallel thread).

Warner