During FlutterMacOS.framework engine universal fat framework creation, make sure the framework is `u=rwx,go=rx` when it's created.

There's a framework-side workaround in-flight https://github.com/flutter/flutter/pull/148580. 

However, this is still a good thing to do engine-side, particularly for add-to-app when they don't go through the `flutter assemble` tooling.

I'll also add a test around here, once this rolls https://github.com/flutter/flutter/blob/02a6c91e4d37d28f42c8f8e4d4335b0defed41c1/packages/flutter_tools/test/host_cross_arch.shard/macos_content_validation_test.dart#L30

Fixes https://github.com/flutter/flutter/issues/148354

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style