bootstrap: add an rpath to libLLVM for the distributed rust-objcopy

[danieljhkim]

Fixes #164063.

rust-objcopy is a renamed copy of llvm-objcopy, shipped in the rustc component at lib/rustlib/$host/bin/. When LLVM is linked dynamically, the copy keeps LLVM's default rpath, $ORIGIN/../lib (@loader_path/../lib on darwin). That resolves to lib/rustlib/$host/lib/, but the rustc component ships libLLVM only in lib/. So rust-objcopy fails to load unless rustup's LD_LIBRARY_PATH/DYLD_FALLBACK_LIBRARY_PATH override is in effect or the llvm-tools component is installed. rustc runs it to strip debuginfo, and it reports a failed strip only as a warning, so release binaries silently keep their debuginfo.

#163954 fixed the same problem for rust-lld in Lld::run. rust-objcopy is built by the Llvm step instead, so that fix doesn't reach it. This PR adds the same rpath entry, $ORIGIN/../../../ on Linux and @loader_path/../../../ on darwin, to the executable linker flags of the main LLVM build, behind the same rpath_enabled/use_host_linker guards. The extra entry only adds a search path for the other LLVM tools: in llvm-tools, those still resolve through rustlib/$host/lib.

Observed on 1.98.1 outside rustup's overrides:

• aarch64-apple-darwin: dyld aborts with Library not loaded: @rpath/libLLVM.dylib (SIGABRT). This is a regression since Link LLVM dynamically on aarch64-apple-darwin #157205.
• x86_64-unknown-linux-gnu: libLLVM.so.22.1-rust-1.98.1-stable: cannot open shared object file. The same $ORIGIN/../lib-only runpath is present back to at least 1.89.

Testing

• cargo check of bootstrap and rustfmt --check pass.
• I validated the rpath value by hand on a copy of the 1.98.1 darwin toolchain layout. rust-objcopy exits with SIGABRT as shipped, and works after install_name_tool -add_rpath @loader_path/../../../ and an ad-hoc re-sign.
• I have not run a full ./x dist locally. Like fix(bootstrap/darwin): fix rpath for distributed LLD #163954, there doesn't seem to be a dist test that covers this, so CI artifacts are the real check.

The download-ci-llvm-stamp is bumped because CI LLVM artifacts change.

cc @rami3l @sgasho

_{Disclosure: the diagnosis, patch and description were produced with an LLM (Claude). I reviewed them and checked the observations above on my own machines.}

🤖 Generated with Claude Code

[rustbot]

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

[rustbot]

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[clubby789]

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