Support "cross-compilation" of Releases?

[tsloughter]

Since we'll be supporting binary installs we could add support for "cross-compilation" -- building releases that target a host not the same as the one you are running on by supporting installing erts from other platforms, similar to rustup supports adding targets: https://rust-lang.github.io/rustup/cross-compilation.html

Obvious issue is the release would still compile any NIF's for the host... But since rebar3 supports (and I think mix) pointing at alternative erts this may be a useful feature if people do use that option.

I guess need to find out if people use that feature of rebar3/mix before spending time on this.