Rollup merge of #145763 - Kobzol:llvm-bindir-cross, r=Mark-Simulacrum

Ship LLVM tools for the correct target when cross-compiling

The LLVM config returned from the `Llvm` step in bootstrap was always the *host* LLVM config (as we cannot execute the cross-compiled LLVM config). But this wasn't obvious in bootstrap before (there was just a comment about it, but that's it), which caused a bug where bootstrap was copying LLVM tools from the host target to the cross-compiled rustc sysroot. This was probably happening for quite a long time, we just haven't noticed before.

Note that I consider this to be mostly a hotfix, I plan to refactor the LLVM handling in bootstrap soon-ish to make it harder to misuse and be better in general.

Fixes: rust-lang/rust#145699

Author: jhpratt