AArch64: use indivisible cmpxchg for 128-bit atomic loads at O0

Like normal atomicrmw operations, at -O0 the simple register-allocator can
insert spills into the LL/SC loop if it's expanded and visible when regalloc
runs. This can cause the operation to never succeed by repeatedly clearing the
monitor. Instead expand to a cmpxchg, which has a pseudo-instruction for -O0.