Find a way to enable overflow checks in distributed builds

[RalfJung]

Currently, overflow checks are disabled in the builds we distribute via rustup. To mitigate this we have the clippy::integer_arithmetic lint enabled in parts of Miri (since #2441), but that is mostly just a verbose way of doing overflow checks by hand. It'd be better if we could find some way to enable overflow checks in the distributed builds, but I am not sure how to do that -- we'd basically want them to be enabled only for the Miri lib crate, but cargo only supports enabling them for all crates involved in the build.