llvm: support cross compilation with useLLVM flag

You can build (partially) with LLVM toolchain using the useLLVM flag.
This works like so:

  nix-build -A hello --arg crossSystem '{ system =
    "aarch64-unknown-linux-musl"; useLLVM = true }'

also don’t separate debug info in lldClang

It doesn’t work currently with that setup hook. Missing build-id?