Distribute cg_clif as rustup component on the nightly channel
This makes it possible to use cg_clif using:
```bash
$ rustup component add rustc-codegen-cranelift-preview --toolchain nightly
$ RUSTFLAGS="-Zcodegen-backend=cranelift" cargo +nightly build
```
cc https://github.com/rust-lang/compiler-team/issues/405.
r? `@Mark-Simulacrum`