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/builder.sh

@@ -6,20 +6,15 @@ done
 
 mkdir $out
 
-# Buid the setup script
 echo "export SHELL=$shell" > $out/setup
 echo "initialPath=\"$initialPath\"" >> $out/setup
+echo "defaultNativeBuildInputs=\"$defaultNativeBuildInputs\"" >> $out/setup
 echo "$preHook" >> $out/setup
 cat "$setup" >> $out/setup
 
 # Allow the user to install stdenv using nix-env and get the packages
 # in stdenv.
-mkdir -p "$out/nix-support"
-echo '# Hack to induce runtime dependencies on the default inputs' \
-    > "$out/nix-support/default-inputs.txt"
-printf '%s\n' $defaultNativeBuildInputs $defaultBuildInputs \
-    >> "$out/nix-support/default-inputs.txt"
+mkdir $out/nix-support
 if [ "$propagatedUserEnvPkgs" ]; then
-    printf '%s ' $propagatedUserEnvPkgs \
-        > "$out/nix-support/propagated-user-env-packages"
+    printf '%s ' $propagatedUserEnvPkgs > $out/nix-support/propagated-user-env-packages
 fi