treewide: Get rid of cc.gcc

Instead, we have a new `gccForLibs`, which has the appropriate
definition for e.g. avoiding `x86_32` linking problems.

pkgs/development/compilers/llvm/6/clang/default.nix

@@ -4,7 +4,6 @@
 }:
 
 let
-  gcc = if stdenv.cc.isGNU then stdenv.cc.cc else stdenv.cc.cc.gcc;
   self = stdenv.mkDerivation ({
     pname = "clang";
     inherit version;
@@ -77,8 +76,6 @@ let
     passthru = {
       isClang = true;
       inherit llvm;
-    } // stdenv.lib.optionalAttrs stdenv.targetPlatform.isLinux {
-      inherit gcc;
     };
 
     meta = {