cross-compilation support

This is punted on in the current version, pending some evaluation of the options. There are ~~three~~ four options I know:

* [cross-rs](https://github.com/cross-rs/cross) (established, based on docker)
* [cargo-zigbuild](https://github.com/rust-cross/cargo-zigbuild) (new kid on the block, [has some blocking issues](https://github.com/rust-cross/cargo-zigbuild/blob/5de2f13fbc1d92a9d732f26af1ca15c0688e2472/README.md#caveats))
* [cargo-xwin](https://github.com/rust-cross/cargo-xwin) (very specific to targetting windows? just learned about this.)
* Do It Our Dang Selves With Rustup (please god no)

This is a blocker for musl and aarch64 (arm64) support.

Note that [taiki-e/upload-rust-binary-action](https://github.com/taiki-e/upload-rust-binary-action) *does* handle this and in my experiments whatever it does Just Works (great work from taiki-e as always).

No repository files or tests are identified. Evaluate cross-rs, cargo-zigbuild, cargo-xwin, and rustup against the musl and aarch64 requirements, using taiki-e/upload-rust-binary-action as a working reference. The issue is done when an approach is selected and cross-compilation support for those targets is implemented.

Written by the indexing model from the issue text.