patch `rust-lld` and `ld.lld` on NixOS When `rustc` uses its self-contained lld, we also need to patch `rust-lld` and `ld.lld`. The `rpath` for `rust-lld` is `$ORIGIN/../../../:$ORIGIN/../lib`, so I use `--add-rpath` instead of `--set-rpath`, which should be easier to maintain. I also changed `src/bootstrap/src/core/download.rs`, even this doesn't fix any known issues. For the `lld-wrapper.sh` of lld, refer to: https://github.com/rust-lang/rustc-dev-guide/pull/1999.