message: fix a comment now that we have concurrency
diff: @@ -364,7 +364,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  | "atomic_singlethreadfence"
  => {
  let &[] = check_arg_count(args)?;
  - // we are inherently singlethreaded and singlecored, this is a nop
  + // FIXME: this will become relevant once we try to detect data races.
  }
  _ if intrinsic_name.starts_with("atomic_xchg") => {
  