gcc: Fix after merge

- NIX_CC_CROSS is now completely gone!

- NIX_CC is defined reliably, so no manual def needed

- stdenv.ccCross -> stdenv.cc, also removing need for "or" fallback

pkgs/development/compilers/gcc/6/default.nix

@@ -35,6 +35,7 @@
 , cloog # unused; just for compat with gcc4, as we override the parameter on some places
 , darwin ? null
 , buildPlatform, hostPlatform, targetPlatform
+, buildPackages
 }:
 
 assert langJava     -> zip != null && unzip != null
@@ -342,7 +343,7 @@ stdenv.mkDerivation ({
       )
     }
     ${optionalString (!(crossMingw && crossStageStatic))
-      "--with-native-system-header-dir=${getDev (stdenv.ccCross or stdenv.cc).libc}/include"}
+      "--with-native-system-header-dir=${getDev stdenv.cc.libc}/include"}
     ${if langAda then " --enable-libada" else ""}
     ${if targetPlatform == hostPlatform && targetPlatform.isi686 then "--with-arch=i686" else ""}
     ${if targetPlatform != hostPlatform then crossConfigureFlags else ""}
@@ -407,8 +408,7 @@ stdenv.mkDerivation ({
     buildFlags = "";
   };
 
-  NIX_BUILD_CC = stdenv.cc;
-  NIX_CC_CROSS = stdenv.ccCross or null;
+  NIX_BUILD_CC = buildPackages.stdenv.cc;
 
   # Needed for the cross compilation to work
   AR = "ar";