1.0.5.2: non-racy WITH-SPINLOCK-AND-WITHOUT-GCING

1.0.5.2: non-racy WITH-SPINLOCK-AND-WITHOUT-GCING

 * It used to be possible for an interrupt or a GC request to come
   in during the small window after RECEIVE-PENDING-INTERRUPTS, but
   while we were running with interrupts and GC disabled. This would
   leave it pending and block further ones -- without any guarantee
   when the next one would come in.

   Experimentally this could also deadlock GC, but the code path
   leading to that is not entirely clear.

 * WITHOUT-GCING can be implemented by using just a single UWP, so do
   it like that.