As per Carlo Kok on Zulip #cranelift, this breaks builds with stable Rust pre-1.43, as `core::u8::MAX` was only stabilized then. We'd like to support older versions if we can easily do so. This PR also adds `cranelift-tools` to the crates checked on CI with Rust 1.41.0, which pulls in all backends (including `aarch64`).