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?

pkgs/stdenv/cross/default.nix

@@ -54,6 +54,8 @@ in lib.init bootStages ++ [
              then buildPackages.darwin.iosSdkPkgs.clang
            else if crossSystem.useAndroidPrebuilt or false
              then buildPackages."androidndkPkgs_${crossSystem.ndkVer}".gcc
+           else if crossSystem.useLLVM or false
+             then buildPackages.llvmPackages_7.lldClang
            else buildPackages.gcc;
 
       extraNativeBuildInputs = old.extraNativeBuildInputs