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/tools/build-managers/cmake/2.8.nix

@@ -1,6 +1,5 @@
 { stdenv, fetchurl, fetchpatch, curl, expat, zlib, bzip2
 , useNcurses ? false, ncurses, useQt4 ? false, qt4, ps
-, buildPlatform, hostPlatform
 }:
 
 with stdenv.lib;
@@ -35,7 +34,7 @@ stdenv.mkDerivation rec {
     })] ++
     # Don't search in non-Nix locations such as /usr, but do search in our libc.
     [ ./search-path.patch ] ++
-    optional (hostPlatform != buildPlatform) (fetchurl {
+    optional (stdenv.hostPlatform != stdenv.buildPlatform) (fetchurl {
       name = "fix-darwin-cross-compile.patch";
       url = "https://public.kitware.com/Bug/file_download.php?"
           + "file_id=4981&type=bug";

pkgs/development/tools/build-managers/meson/default.nix

@@ -1,4 +1,4 @@
-{ lib, python3Packages, stdenv, targetPlatform, writeTextDir, substituteAll }:
+{ lib, python3Packages, stdenv, writeTextDir, substituteAll }:
 
 python3Packages.buildPythonApplication rec {
   version = "0.46.1";
@@ -57,10 +57,10 @@ python3Packages.buildPythonApplication rec {
     needs_exe_wrapper = true
 
     [host_machine]
-    system = '${targetPlatform.parsed.kernel.name}'
-    cpu_family = '${targetPlatform.parsed.cpu.family}'
-    cpu = '${targetPlatform.parsed.cpu.name}'
-    endian = ${if targetPlatform.isLittleEndian then "'little'" else "'big'"}
+    system = '${stdenv.targetPlatform.parsed.kernel.name}'
+    cpu_family = '${stdenv.targetPlatform.parsed.cpu.family}'
+    cpu = '${stdenv.targetPlatform.parsed.cpu.name}'
+    endian = ${if stdenv.targetPlatform.isLittleEndian then "'little'" else "'big'"}
   '';
 
   # 0.45 update enabled tests but they are failing