compiler/{sparc,ppc}/macros: with-pinned-objects improvements.

compiler/{sparc,ppc}/macros: with-pinned-objects improvements.

  * For all precise gencgc backends, with-pinned-objects uses an
explicit "pin list".  This pin list should be stack-allocated.

  * Declare the pin list to be TRULY-DYNAMIC-EXTENT, for both
backends.  This won't actually do anything unless the backend
also supports :stack-allocatable-fixed-objects or more than two
objects are to be pinned at once (one-arg LIST and two-arg LIST*
are both converted to CONS by the compiler, and CONS falls under
:stack-allocatable-fixed-objects rather than
:stack-allocatable-lists).