netbsd: switch to the split GCC package set

Make `useGccNG` the default for every NetBSD platform, as Cygwin's already
is, rather than opting in one example system. NetBSD 11.0 is only supported
with the split set, so this comes straight after the version bump: the
monolithic `gcc` is not kept working for it.

NetBSD is low tier and has to use GCC rather than LLVM, so it is a good
place to dogfood the split set.

With the fixes that follow, GCC 16 builds `hello`, `zlib`, `bash`,
`perl`, `python3` and the NetBSD libc for it.

`x86_64-netbsd-llvm` is unaffected, since `useLLVM` takes precedence.

Assisted-by: Claude Code (Claude Opus 5.5)

lib/systems/default.nix

@@ -163,10 +163,9 @@ let
         # Use the split GCC package set (`gccNGPackages`) instead of the
         # monolithic `gcc`.
         #
-        # I (@Ericson2314) plan on making more obscure low-tier
-        # platforms (e.g. NetBSD) use it soon, so we can dogfood GCC NG
-        # and thereby iron out its bugs.
-        useGccNG = final.isCygwin;
+        # NetBSD is here to dogfood it: it is low tier and has to use GCC
+        # rather than LLVM.
+        useGccNG = final.isCygwin || final.isNetBSD;
 
         libc =
           if final.isDarwin then