Always call freeze_support() if sys.frozen is True

This shouldn't be necessary, but in practice macOS also requires
freeze_support() so let's play it safe and always call it when
sys.frozen is truthy. Only a tiny sliver of users will be using these
PyInstaller executables and the start-up performance is already terrible
as the executable unzips itself on the fly anyway.

src/black/__init__.py

@@ -1375,7 +1375,7 @@ def patch_click() -> None:
 
 
 def patched_main() -> None:
-    if sys.platform == "win32" and getattr(sys, "frozen", False):
+    if getattr(sys, "frozen", False):
         from multiprocessing import freeze_support
 
         freeze_support()