Add double-width compare-and-exchange instructions for x86oid.

Also add a test for the CPUID instruction.

There are new private functions for the enlightened.
These could be used as the implementation of a publicized CAS
expander for something like
  (CAS (car+cdr my-cons) ...) ; Don't do this (not implemented)
but I didn't think that "shoehorning" these into CAS was better
than just providing the functions.
Improve LOGBITP of constant 2nd arg