Rust: Add experimental support with Ninja

The TU for Rust is a crate, not per-source.  `rustc` expects entry file
to TU as input, and cannot work with object files.  So we need to have
one-step build/link in most cases, similar to what old Swift was doing.

Issue: #25492
Signed-off-by: Ayush Singh <ayush@beagleboard.org>
34 files changed