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.
Edited by Administrator