When `target.$triple.runner` is specified, it will be used for any execution commands by cargo including `cargo run`, `cargo test` and `cargo bench`. The original file is passed to the runner executable as a first argument.

This allows to run tests when cross-comping Rust projects.

This is not a complete solution, and might be extended in future for better ergonomics to support passing extra arguments to the runner itself or overriding runner from command line, but it should already unlock major existing usecases.

Fixes rust-lang#1411
Resolves rust-lang#3626