Auto merge of #3954 - RReverser:run-with, r=alexcrichton

Add support for custom target-specific runners

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 the command line, but it should already unlock major existing use cases.

Fixes #1411
Resolves #3626