lib.systems: build Cygwin with the split GCC package set

Cygwin is the first platform to select `useGccNG`, and it is the natural
one: its libc is partly C++, so building `newlib-cygwin` needs C++ headers
before any libc exists. The monolithic compiler handles that with a
Cygwin-specific `langCC = stdenv.targetPlatform.isCygwin`; the split set
has a stage that provides them, `libstdcxx-no-libc`, which the preceding
commits add and wire in.

Derived from the platform rather than set on the example spec, so it holds
for any Cygwin triple, alongside the `useLLVM` rule it sits under. The
comment saying no platform selects it is retired here, since that is what
this changes.

This changes what `pkgsCross.x86_64-cygwin` builds with for everyone, not
only for callers passing the flag. Verified by building
`gccNGPackages.{libgcc,libstdcxx,libstdcxx-no-libc}` and `stdenv.cc` with
no `useGccNG` in the invocation; the derivations come out identical to
those built with the flag passed explicitly.

Assisted-by: Claude Code (Claude Opus 5)

lib/systems/default.nix

@@ -161,14 +161,12 @@ let
             (with final; isWindows && isAarch64);
 
         # Use the split GCC package set (`gccNGPackages`) instead of the
-        # monolithic `gcc`. No platform selects it yet; it is opt-in, set
-        # explicitly on a platform spec, so that the split set can be exercised
-        # before anything depends on it.
+        # monolithic `gcc`.
         #
-        # I (@Ericson2314) plan on making obscure low-tier platforms (e.g.
-        # NetBSD) use it soon, so we can dogfood GCC NG and thereby iron out its
-        # bugs.
-        useGccNG = false;
+        # 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;
 
         libc =
           if final.isDarwin then