1.0.41.15: gencgc: Implement object pinning for non-x86oids.

  * Add a new static symbol for all non-x86oid gencgc targets,
*pinned-objects*.

  * Bind *pinned-objects* to NIL during thread creation if it is
defined.

  * During garbage collection, where x86oids preserve pointers on
the various thread stacks, use the same mechanism to preserve
pointers linked on *pinned-objects* for each thread.

  * Change the PPC version of with-pinned-objects to rebind
*pinned-objects* with the new objects to pin prepended when using
gencgc.