stdenv-setup: Ease the transition with native builds

- All deps go on the PATH

 - CC and Bintools wrappers with their host != depender's host still get their
   setup hooks run.

 - Environment hooks get applied to all packages

This isn't so elegent, but eases the transition on a very significant
PR.

pkgs/build-support/bintools-wrapper/setup-hook.sh

@@ -4,8 +4,11 @@
 
 set -u
 
-# Skip setup hook if we're not a build-time dep
-(( "$hostOffset" < 0 )) || return 0
+# Skip setup hook if we're neither a build-time dep, nor, temporarily, doing a
+# native compile.
+#
+# TODO(@Ericson2314): No native exception
+[[ -z ${crossConfig-} ]] || (( "$hostOffset" < 0 )) || return 0
 
 bintoolsWrapper_addLDVars () {
     case $depHostOffset in