stdenvs: Distinguish between extraBuildInputs and extraNativeBuildInputs

Additionally, instead of pulling them from `setup.sh`, route them via
Nix. This gets us one step closer to making stdenv be a plain attribute
set instead of a derivation.

pkgs/stdenv/native/default.nix

@@ -66,7 +66,7 @@ let
     export lt_cv_deplibs_check_method=pass_all
   '';
 
-  extraBuildInputsCygwin = [
+  extraNativeBuildInputsCygwin = [
     ../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;
 
-      extraBuildInputs =
-        if system == "i686-cygwin" then extraBuildInputsCygwin else
-        if system == "x86_64-cygwin" then extraBuildInputsCygwin else
+      extraNativeBuildInputs =
+        if system == "i686-cygwin" then extraNativeBuildInputsCygwin else
+        if system == "x86_64-cygwin" then extraNativeBuildInputsCygwin else
         [];
 
       initialPath = extraPath ++ path;