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/libraries/boehm-gc/default.nix

@@ -1,6 +1,5 @@
 { lib, stdenv, fetchurl, fetchpatch, pkgconfig, libatomic_ops
 , enableLargeConfig ? false # doc: https://github.com/ivmai/bdwgc/blob/v7.6.6/doc/README.macros#L179
-, buildPlatform, hostPlatform
 }:
 
 stdenv.mkDerivation rec {
@@ -30,7 +29,7 @@ stdenv.mkDerivation rec {
     sha256 = "1gydwlklvci30f5dpp5ccw2p2qpph5y41r55wx9idamjlq66fbb3";
   }) ] ++
     # https://github.com/ivmai/bdwgc/pull/208
-    lib.optional hostPlatform.isRiscV ./riscv.patch;
+    lib.optional stdenv.hostPlatform.isRiscV ./riscv.patch;
 
   configureFlags =
     [ "--enable-cplusplus" ]
@@ -40,7 +39,7 @@ stdenv.mkDerivation rec {
   doCheck = true; # not cross;
 
   # Don't run the native `strip' when cross-compiling.
-  dontStrip = hostPlatform != buildPlatform;
+  dontStrip = stdenv.hostPlatform != stdenv.buildPlatform;
 
   enableParallelBuilding = true;