1.0.41.35: ppc: Implement compare-and-swap-vops.

  * 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.