Use Xamarin.Mac.dll on mono net4x profile

[atsushieno]

I had been exploring any possibility to use Xamarin.Mac class libraries on .NET Core, and instead I found that it seems that Xamarin.Mac implementation is available when Mono runtime is used: https://docs.microsoft.com/en-us/xamarin/mac/app-fundamentals/console

Unlike .NET Core where the runtime internal API and embedded support is completely different, external mono runtime would totally work if everything is handled at libxammac.dylib and co.

This resolves most of my use cases on Mac desktop, so I would try to make changes to our Mac desktop (net4x) support to make use of it.