Rollup merge of #144659 - mati865:gnullvm-vendor-libunwind, r=Kobzol

bootstrap: refactor mingw dist and fix gnullvm

Fixes rust-lang/rust#144533

The first two commits are NFC and only clean up the code, paving the way for the third commit. That said, I think they are worthwhile even without that fix - reusing the same function for two different outcomes was confusing.

The third commit is the fix for rust-lang/rust#144533, but due to the cross-compilation dance it requires a workaround to find the DLL since that logic really was meant only for Windows builders. That workaround is short-lived and will be removed as soon as gnullvm bootstraps itself.

Author: Zalathar