Function llvm_lib_rpath 

fn llvm_lib_rpath(
    builder: &Builder<'_>,
    target: TargetSelection,
) -> Option<&'static str>

Generate a RPATH entry for a binary that will link to LLVM (libLLVM.so) dynamically. This will make it possible to execute the binary outside Rustup, so that it can still find the libLLVM.so file.