ppc support for stack-allocatable-vectors

ppc support for stack-allocatable-vectors

  * This turned out to be fairly straightforward.  Unlike in a
heap-allocation-only regime, where a VOP is required to :TRANSLATE
ALLOCATE-VECTOR, the :STACK-ALLOCATABLE-VECTORS feature enables an
LTN-ANNOTATE optimizer for ALLOCATE-VECTOR that substitutes
invocations of one of two named VOPs.

  * To convert from the old regime to the new, rename the old VOP
to fit the new naming scheme, and write a new VOP to do the stack
allocation.

  * As a cleaning-up-a-loose-end matter, lose the :TRANSLATE
option for the old VOP.

  * And as a "being somewhat cute about things" matter, make the
support for stack-allocatable-vectors selectable at build time,
which should provide a quick overview of how to make this work on
some other platform, should anyone else be interested later on.