For Unified, set XAMCORE_2_0, UNIFIED and __UNIFIED__.
For Xamarin.iOS/tvOS/watchOS set both the normal and underscored versions
(IOS and __IOS__, TVOS and __TVOS__, and WATCHOS and __WATCHOS__).
The underscored versions are the public symbols we're setting in the
corresponding projects, so we should use those everywhere to simplify our
code, but due to historical reasons we're still using the other variants in
existing code.
Making sure all the possible variants are set for all projects, makes it
possible to only use the underscored versions in new code.
Also define `GENERATOR` for the generator, so that we can easily share
files between the generator and platform assemblies.
Rolf Bjarne Kvinge