+universal for x864+arm64 on Macintel

[René J.V. Bertin]

>Base is exactly smart enough. When supported_archs contains only 1 arch, it does make sense to offer a universal variant, therefore base prevents it.

As I said before, supported_archs contains x86_64 and arm64.

If ports should be able to create a universal variant in case that variant isn't created automatically, "base" should be polite enough to assume that the port maintainers know and have a reason for what they're doing.