Good question! The TL;DR as I understand it is that it won't matter too much. For example, the self-hosted x86_64 backend (which is coincidentally becoming default for debugging on linux right now - https://github.com/ziglang/zig/pull/24072) has full support for most (all?) builtins. I don't think that we need to worry about that.

It's an interesting question about how Zig will handle additional builtins and data representations. The current way I understand it is that there's an additional opt-in translation layer that converts unsupported/complicated IR to IR which the backend can handle. This is referred to as the compiler's "Legalize" stage. It should help to reduce this issue, and perhaps even make backends like https://github.com/xoreaxeaxeax/movfuscator possible :)