[UIKit] Add a custom version of UIDevice.CheckSystemVersion to Mac Catalyst. Fixes #10453.

Add a custom version of UIDevice.CheckSystemVersion that checks the iOS version for
Mac Catalyst.

Fixes https://github.com/xamarin/xamarin-macios/issues/10453.

src/ObjCRuntime/Runtime.cs

@@ -1806,6 +1806,21 @@ namespace ObjCRuntime {
 		{
 			return GCHandle.ToIntPtr (GCHandle.Alloc (value));
 		}
+
+#if __MACCATALYST__
+		static string _iOSSupportVersion;
+		internal static string iOSSupportVersion {
+			get {
+				if (_iOSSupportVersion == null) {
+					// This is how Apple does it: https://github.com/llvm/llvm-project/blob/62ec4ac90738a5f2d209ed28c822223e58aaaeb7/lldb/source/Host/macosx/objcxx/HostInfoMacOSX.mm#L100-L105
+					using (var dict = NSMutableDictionary.FromFile ("/System/Library/CoreServices/SystemVersion.plist")) {
+						_iOSSupportVersion = dict.ObjectForKey ((NSString) "iOSSupportVersion").ToString ();
+					}
+				}
+				return _iOSSupportVersion;
+			}
+		}
+#endif
 	}
 	
 	internal class IntPtrEqualityComparer : IEqualityComparer<IntPtr>

src/UIKit/UIDevice.cs

@@ -16,6 +16,8 @@ namespace UIKit {
 		{
 #if WATCH
 			return Runtime.CheckSystemVersion (major, minor, WatchKit.WKInterfaceDevice.CurrentDevice.SystemVersion);
+#elif __MACCATALYST__
+			return Runtime.CheckSystemVersion (major, minor, Runtime.iOSSupportVersion);	
 #else
 			return Runtime.CheckSystemVersion (major, minor, SystemVersion);
 #endif