Since 8172144, Implement "JEP 270: Reserved Stack Areas for Critical
Sections" and 8173339, Fix minimum stack size computations, the stack
bang size changed to 25 4k pages. Unfortunately, this means that we
now generate three instructions for a stack bang.

We should change the instruction sequence we use for stack bangs from

  mov x9, #0xffffffffffffc000
  movk x9, #0xfffe, lsl #16
  str xzr, [sp,x9]

to

  sub x9, sp, #0x14, lsl #12
  str xzr, [x9]