Add -lzstd to RT_LIBS and when USE_SYSTEM_LLVM=1 (#59475)
Rather than depending on `llvm-config --system-libs` to link libzstd to
libjulia-internal, we should ask for it explicitly, since we now depend
on it even if LLVM was not configured with zstd.
Possibly fixes the error in
https://github.com/JuliaPackaging/Yggdrasil/pull/11999#issuecomment-3251050856
