Upstream here!
LLVM Rust toolchain is still a little immature and code generation for other architectures is lacking
Do you have bug reports? Firefox has been shipping on these architectures with Rust code for years now.
We have been building Rust-coreutils on Debian & Ubuntu for a few years too:
https://buildd.debian.org/status/package.php?p=rust-coreutils
https://launchpad.net/ubuntu/+source/rust-coreutils/0.0.27-3