wrapBintoolsWith: fix final stage NetBSD

Fixes "bintools" on NetBSD -- without this only the version in stdenv
would work, not the one from the final stage.

Andi tried to warn me about this[1].  Andi, I'm sorry for doubting you.

[1]: https://github.com/NixOS/nixpkgs/pull/124499#discussion_r641949801

pkgs/build-support/bintools-wrapper/default.nix

@@ -14,7 +14,9 @@
   if libc == null then
     null
   else if stdenvNoCC.targetPlatform.isNetBSD then
-    if libc != targetPackages.netbsdCross.headers then
+    if !(targetPackages ? netbsdCross) then
+      netbsd.ld_elf_so
+    else if libc != targetPackages.netbsdCross.headers then
       targetPackages.netbsdCross.ld_elf_so
     else
       null