* Based roughly on the x86-64 version and the differences
between x86oid define-full-reffer and the PPC use of VOPs
instead (the VOPs seem smart enough, why do x86oids do the
whole define-full-reffer / -c VOP thing?).
* Compare-and-swap VOPs are full memory barriers.
* Left the VOPs conditionally-compiled in based on the
compare-and-swap-vops feature, more as documentation of what
is involved than anything else.
* Enabled compare-and-swap-vops for all PPC targets.