LLVM compiles C, C++, Rust, etc. into an intermediate language and then compiles that language into assembly for the target platform. I’m not sure if gcc uses an intermediate language or not. Either way, the compiler can compile any of its supported languages into any of its target platforms. For Rust, you will probably need to look into “no_std” for systems that don’t have a typical libc setup.

I emulate a lot of old games on my Steam Deck. It’s not too hard, but requires some work. I will do the work for non-Steam stores if/when there is something I want to play from one. However, I suspect not releasing your games on Steam will really limit your reach. My guess is that most people won’t go through the effort to get itch.io games work on thier Deck.