Add FIXME about type of `addr` in futex implementation.
Add FIXME about type of `addr` in futex implementation.

Co-authored-by: Ralf Jung <9b466094ec991a03cb95c489c19c4d75635f0ae5@ralfj.de>
rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri