Move Miri extern functions to a Miri utils crate

[dtolnay]

These things: https://github.com/rust-lang/miri/tree/73452b3213b906a7d2a08f176d38730bcc363f6f#miri-extern-functions

Quoting @saethlin in #2757 (comment):

We should really figure out some kind of Miri utils crate if we're going to keep incentivizing library authors to do stuff like this. As-written, this is ugly to use and also the mechanism that this hooks into makes me uncomfortable because it's so permissive.

[dtolnay]

I confirmed that [target.'cfg(miri)'.dependencies] in Cargo.toml works correctly, those dependencies will build only during cargo miri and not for ordinary use of the library. It should be documented that this is the intended way to declare the dependency on the miri utils.

[bjorn3]

Another option would be for miri to provide this utils crate in the sysroot such that #[cfg(miri)] extern crate miri_utils; works and always pulls in the right utils crate version for the current miri version.