deps: Link LLVM tools with an rpath-link to Julia's library directory
When building LLVM from source, zlib and zstd are taken from Julia's
build prefix, giving libLLVM.so NEEDED entries on the libraries there.
GNU ld does not consult -L paths when resolving a shared library's
transitive dependencies, so linking LLVM's own tools against libLLVM
fails under a sysroot'ed cross-toolchain (as used in the package_linux
CI rootfs) whose sysroot lacks libzstd and ships a zlib without the
ZLIB_1.2.0 versioned symbols. Pass -rpath-link alongside the existing
-L workaround so these libraries resolve from the build tree.
Fixes the source-LLVM coverage build in
https://buildkite.com/julialang/julia-pr/builds/1601#01a010d2-6ae3-45fc-9d49-81246bb5a704
Assisted-by: Claude Code (Fable 5)