MFC arch.7 man page describing some arch-specific details

MFC of r318386, r317938, r317844, r317843, r317840, r317835, r317809,

r317790, r317754, r317708, r317681, r312563, r308251, r308130,
r307687, r303153, r303134, r303045, r303044, r303033

Changes by br, emaste, jhb, jhibbits, kevlo, kib

Sponsored by: The FreeBSD Foundation