1.0.18.25: tweak stack allocation on x86 and x86-64

1.0.18.25: tweak stack allocation on x86 and x86-64

 * Use MAYBE-PSEUDO-ATOMIC in the LIST-OR-LIST* VOP: stack allocation
   doesn't need PA.

 * When using STACK-ALLOCATE-P parameter with ALLOCATION, also pass in
   the lowtag. This allows us to generate

     LEA REG [STACK_REG+LOWTAG]

   instead of

     MOV REG STACK_REG
     LEA REG [REG+LOWTAG]

   for stack allocation & tagging.

   On x86-64 can use the same trick in the inline path for heap
   allocation as well.