message: Implement __isb for AArch64
  This is necessary for the mutex implementation of libstd when there is
  contention. Seems like I hadn't hit this before by sheer luck.
diff: @@ -16,6 +16,10 @@ pub(crate) fn codegen_aarch64_llvm_intrinsic_call<'tcx>(
  // llvm.aarch64.neon.sqshl.v*i*
  match intrinsic {
  + "llvm.aarch64.isb" => {
  + fx.bcx.ins().fence();
  + }
  +
  _ if intrinsic.starts_with("llvm.aarch64.neon.abs.v") => {
  intrinsic_args!(fx, args => (a); intrinsic);
  