message: stack address functions for macOS
diff: @@ -544,16 +544,21 @@ impl<'a, 'mir, 'tcx: 'mir + 'a> EvalContextExt<'tcx, 'mir> for EvalContext<'a, '
  // Determining stack base address
  "pthread_attr_init" | "pthread_attr_destroy" | "pthread_attr_get_np" |
  - "pthread_getattr_np" | "pthread_self" => {
  + "pthread_getattr_np" | "pthread_self" | "pthread_get_stacksize_np" => {
  self.write_null(dest)?;
  }
  "pthread_attr_getstack" => {
  // second argument is where we are supposed to write the stack size
  let ptr = self.ref_to_mplace(self.read_value(args[1])?)?;
  - self.write_scalar(Scalar::from_int(0x80000, args[1].layout.size), ptr.into())?;
  + let stackaddr = Scalar::from_int(0x80000, args[1].layout.size); // just any address
  + self.write_scalar(stackaddr, ptr.into())?;
  // return 0
  self.write_null(dest)?;
  }
  + "pthread_get_stackaddr_np" => {
  + let stackaddr = Scalar::from_int(0x80000, dest.layout.size); // just any address
  + self.write_scalar(stackaddr, dest)?;
  + }
  // Stub out calls for condvar, mutex and rwlock to just return 0
  "pthread_mutexattr_init" | "pthread_mutexattr_settype" | "pthread_mutex_init" |
  