treewide: Remove usage of remaining redundant platform compatability stuff

Want to get this out of here for 18.09, so it can be deprecated
thereafter.

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

@@ -1,7 +1,7 @@
 { stdenv, targetPackages, fetchurl, noSysDirs
 , langC ? true, langCC ? true, langFortran ? false
-, langObjC ? targetPlatform.isDarwin
-, langObjCpp ? targetPlatform.isDarwin
+, langObjC ? stdenv.targetPlatform.isDarwin
+, langObjCpp ? stdenv.targetPlatform.isDarwin
 , langJava ? false
 , langGo ? false
 , profiledCompiler ? false
@@ -20,15 +20,15 @@
 , libXrandr ? null, libXi ? null, inputproto ? null, randrproto ? null
 , x11Support ? langJava
 , enableMultilib ? false
-, enablePlugin ? hostPlatform == buildPlatform # Whether to support user-supplied plug-ins
+, enablePlugin ? stdenv.hostPlatform == stdenv.buildPlatform # Whether to support user-supplied plug-ins
 , name ? "gcc"
 , libcCross ? null
 , crossStageStatic ? false
 , # Strip kills static libs of other archs (hence no cross)
-  stripped ? hostPlatform == buildPlatform && targetPlatform == hostPlatform
+  stripped ? stdenv.hostPlatform == stdenv.buildPlatform
+          && stdenv.targetPlatform == stdenv.hostPlatform
 , gnused ? null
 , cloog # unused; just for compat with gcc4, as we override the parameter on some places
-, buildPlatform, hostPlatform, targetPlatform
 , buildPackages
 }:
 
@@ -40,7 +40,7 @@ assert langJava     -> zip != null && unzip != null
 assert libelf != null -> zlib != null;
 
 # Make sure we get GNU sed.
-assert hostPlatform.isDarwin -> gnused != null;
+assert stdenv.hostPlatform.isDarwin -> gnused != null;
 
 # The go frontend is written in c++
 assert langGo -> langCC;
@@ -52,6 +52,8 @@ let version = "6.4.0";
 
     enableParallelBuilding = true;
 
+    inherit (stdenv) buildPlatform hostPlatform targetPlatform;
+
     patches =
       [ ../use-source-date-epoch.patch ]
       ++ optional (targetPlatform != hostPlatform) ../libstdc++-target.patch