Provision more target triples

[mcandre]

Some of the available target triples in rustup target list lack corresponding entries in cross-toolchains. Can we get more of these targets going for cross?

[Emilgardis]

This is a matter of need, if there is a target you need a cross-compilation suite for using cross and can provide a image for it, feel free to add it! We won't add t3 targets just like that, and some T3 targets are harder than others to implement.

Here is all known targets: https://doc.rust-lang.org/rustc/platform-support.html, afaik, we support all T1 and most common T2 targets already, either here or in https://github.com/cross-rs/cross