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/development/compilers/llvm/7/clang/default.nix

@@ -5,7 +5,6 @@
 }:
 
 let
-  gcc = if stdenv.cc.isGNU then stdenv.cc.cc else stdenv.cc.cc.gcc;
   self = stdenv.mkDerivation ({
     name = "clang-${version}";
 
@@ -81,8 +80,8 @@ let
     passthru = {
       isClang = true;
       inherit llvm;
-    } // stdenv.lib.optionalAttrs stdenv.targetPlatform.isLinux {
-      inherit gcc;
+    } // stdenv.lib.optionalAttrs (stdenv.targetPlatform.isLinux || (stdenv.cc.isGNU && stdenv.cc.cc ? gcc)) {
+      gcc = if stdenv.cc.isGNU then stdenv.cc.cc else stdenv.cc.cc.gcc;
     };
 
     meta = {