message: Implement core::hint::spin_loop() on AArch64
  This is used in the futex based mutex implementation of libstd
diff: @@ -14,6 +14,10 @@ pub(crate) fn codegen_llvm_intrinsic_call<'tcx>(
  target: Option<BasicBlock>,
  ) {
  match intrinsic {
  + "llvm.x86.sse2.pause" | "llvm.aarch64.isb" => {
  + // Spin loop hint
  + }
  +
  // Used by `_mm_movemask_epi8` and `_mm256_movemask_epi8`
  "llvm.x86.sse2.pmovmskb.128" | "llvm.x86.avx2.pmovmskb" | "llvm.x86.sse2.movmsk.pd" => {
  intrinsic_args!(fx, args => (a); intrinsic);
  