wit-bindgen-rt: add option to regenerate binary artifacts

wit-bindgen-rt is now (since 1.86) part of rustc's transitive dep tree, it would be great if there were an option (e.g., a feature honored by build.rs?) to regenerate the included binary artifacts that could be used as part of building rustc. distros like Debian have this as requirement for distribution.

I can try to whip something up if this seems acceptable.