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/libelf/default.nix

@@ -1,6 +1,5 @@
 { stdenv
 , fetchurl, autoreconfHook, gettext
-, buildPlatform, hostPlatform
 }:
 
 stdenv.mkDerivation rec {
@@ -20,17 +19,17 @@ stdenv.mkDerivation rec {
   configureFlags = []
        # Configure check for dynamic lib support is broken, see
        # http://lists.uclibc.org/pipermail/uclibc-cvs/2005-August/019383.html
-    ++ stdenv.lib.optional (hostPlatform != buildPlatform) "mr_cv_target_elf=yes"
+    ++ stdenv.lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) "mr_cv_target_elf=yes"
        # Libelf's custom NLS macros fail to determine the catalog file extension
        # on Darwin, so disable NLS for now.
-    ++ stdenv.lib.optional hostPlatform.isDarwin "--disable-nls";
+    ++ stdenv.lib.optional stdenv.hostPlatform.isDarwin "--disable-nls";
 
   nativeBuildInputs = [ gettext ]
        # Need to regenerate configure script with newer version in order to pass
        # "mr_cv_target_elf=yes", but `autoreconfHook` brings in `makeWrapper`
        # which doesn't work with the bootstrapTools bash, so can only do this
        # for cross builds when `stdenv.shell` is a newer bash.
-    ++ stdenv.lib.optional (hostPlatform != buildPlatform) autoreconfHook;
+    ++ stdenv.lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) autoreconfHook;
 
   meta = {
     description = "ELF object file access library";