bootstrap: add an rpath to LLVM library for LLVM tools

[danieljhkim]

Closes #164063 by adding rpath to the LLVM tool executables on both Linux and macOS, using the same approach as Lld::run.

This PR is now written by a human, resubmitting #164064 which was diagnosed and written by Claude.

Validation:

macOS

❯ otool -l ~/workspace/repos/rust/build/aarch64-apple-darwin/llvm/bin/llvm-objcopy | grep -A2 LC_RPATH
          cmd LC_RPATH
      cmdsize 40
         path @loader_path/../../../ (offset 12)
--
          cmd LC_RPATH
      cmdsize 32
         path @loader_path/../lib (offset 12)

linux

readelf -d build/x86_64-unknown-linux-gnu/llvm/bin/llvm-objcopy | grep RUNPATH
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/../../../:$ORIGIN/../lib:$ORIGIN/../lib/x86_64-unknown-linux-gnu]

[rustbot]

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @Mark-Simulacrum (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Please see the contribution instructions and our LLM policy for more information.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: bootstrap
• bootstrap expanded to 6 candidates
• Random selection from Mark-Simulacrum, clubby789, jieyouxu

[Kobzol]

r? kobzol

Thanks for the PR. Two remarks:

• We now have the same logic duplicated on three places (OmpOffload, Lld and Llvm). Could you please extract it to a shared function that takes a builderand atarget`? The function should only be called if we link LLVM dynamically.
• Won't the linker flags be also applied to libLLVM.so itself? Have you tried building it on Linux and running lld on it - if yes, what does it print?