Remove Runtime.Arch and ObjCRuntime.Arch from Mac Catalyst, because they don't
apply for a Mac Catalyst app (which is neither a simulator environment, nor a
device environment).
This means that code using these APIs will have to be re-evaluated to
determine what's the correct behavior for Mac Catalyst.
Also update our tests accordingly.
Fixes https://github.com/xamarin/xamarin-macios/issues/10312.
Rolf Bjarne Kvinge
Alex Soto
Sebastien Pouliot