When using libunwind and not building as standalone project, we need to add LLVM library directory to the list of linker directories to ensure it can find libunwind dependency.
This wasn't previously needed since there was only only one unwind target and CMake resolved the name directly. However, with the latest change which allows building both static and shared version of libunwind in one build, that's no longer the case.
This fixes the failure introduced in the latest libunwind change.