Revert "stdenvs: Distinguish between extraBuildInputs and extraNativeBuildInputs"

This reverts commit eeabf85780.

This change suddenly makes tons of stdenv internals visible in
nativeBuildInputs of every derivation, which doesn't seem desirable.
E.g:

````
nix-repl> hello.nativeBuildInputs
[ «derivation /nix/store/bcfkyf6bhssxd2vzwgzmsbn7b5b9rpxc-patchelf-0.9.drv»
  «derivation /nix/store/4wnshnz9wwanpfzcrdd76rri7pyqn9sk-paxctl-0.9.drv»
  << snip 10+ lines >>
  «derivation /nix/store/d35pgh1lcg5nm0x28d899pxj30b8c9b2-gcc-wrapper-6.4.0.drv»
]
````

pkgs/stdenv/native/default.nix

@@ -66,7 +66,7 @@ let
     export lt_cv_deplibs_check_method=pass_all
   '';
 
-  extraNativeBuildInputsCygwin = [
+  extraBuildInputsCygwin = [
     ../cygwin/all-buildinputs-as-runtimedep.sh
     ../cygwin/wrap-exes-to-find-dlls.sh
   ] ++ (if system == "i686-cygwin" then [
@@ -94,9 +94,9 @@ let
         if system == "x86_64-cygwin" then prehookCygwin else
         prehookBase;
 
-      extraNativeBuildInputs =
-        if system == "i686-cygwin" then extraNativeBuildInputsCygwin else
-        if system == "x86_64-cygwin" then extraNativeBuildInputsCygwin else
+      extraBuildInputs =
+        if system == "i686-cygwin" then extraBuildInputsCygwin else
+        if system == "x86_64-cygwin" then extraBuildInputsCygwin else
         [];
 
       initialPath = extraPath ++ path;