Crossdirect: support rust

Add hello-world-rust and update cross/crossdirect to support rust.

Details: https://gitlab.com/postmarketOS/pmbootstrap/issues/1861

Setting to WIP, until it's confirmed to work with squeekboard. squeekboard needs some more work, see #407 (closed). But this MR is useful as-is now, it greatly shortens the compile time and falls back to rustc+qemu in the edge case where it is necessary.

EDIT: Must be tested together with https://gitlab.com/postmarketOS/pmbootstrap/merge_requests/1850.