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/generic/setup.sh

@@ -317,17 +317,18 @@ if [ -z "$crossConfig" ]; then
     # are handled identically to nativeBuildInputs
     declare -a nativePkgs
     for i in $nativeBuildInputs $buildInputs \
+             $defaultNativeBuildInputs $defaultBuildInputs \
              $propagatedNativeBuildInputs $propagatedBuildInputs; do
         findInputs "$i" nativePkgs propagated-native-build-inputs
     done
 else
     declare -a crossPkgs
-    for i in $buildInputs $propagatedBuildInputs; do
+    for i in $buildInputs $defaultBuildInputs $propagatedBuildInputs; do
         findInputs "$i" crossPkgs propagated-build-inputs
     done
 
     declare -a nativePkgs
-    for i in $nativeBuildInputs $propagatedNativeBuildInputs; do
+    for i in $nativeBuildInputs $defaultNativeBuildInputs $propagatedNativeBuildInputs; do
         findInputs "$i" nativePkgs propagated-native-build-inputs
     done
 fi