Sounds like the real issue is that some Git developers have ancient, rigid requirements for their own development machines.
Zig takes the crown on that one, to the point that some people use Zig to cross-compile Go projects with CGo dependencies.
Rust might have a harder time if it wanted a corresponding feature because it doesn't natively build C like Zig does (using libclang). Either it would have to start using libclang or ship with rust re-implementations of the C library. AFAIK it's impossible to write the C++ library in Rust though.
I switched from Go and I feel like Go was much better at this than Rust.
(I tried “cross” but it was very slow and I found it faster to rsync the files inside the container and then run the build scripts)
But, my point is you shouldn't even have to cross-compile Git to a platform like NonStop in order to develop NonStop apps. So the portability of Rust shouldn't even matter here. The app developer should be able to run their Git commands on a supported platform and cross-compile their own app to NonStop.