* Fix many version checks to be based on Xcode version instead of iOS version.
* Added/fixed a few expected values according to platform version to match behavior in older macOS versions.
Enabling CloudKit on watchOS required to remove some default .ctor that
watchOS does not _support_.
This commit fix this by either
* replacing the default .ctor with something that works across all
platforms (best); or
* removing the default .ctor only on watchOS;
The commit also mark as abstract three existing types for watchOS (and
for XAMCORE_4_0) that were found reviewing the bindings.
Rolf Bjarne Kvinge
Sebastien Pouliot
Manuel de la Pena