Convert Runtime.Dispose to use GCHandle.

runtime/delegates.t4

@@ -188,7 +188,7 @@
 		},
 
 		new XDelegate ("void", "void", "xamarin_dispose",
-			"MonoObject *", "IntPtr", "mobj"
+			"GCHandle->MonoObject*", "IntPtr", "mobj"
 		) {
 			WrappedManagedFunction = "Dispose",
 			OnlyDynamicUsage = false,

src/ObjCRuntime/Runtime.cs

@@ -712,9 +712,9 @@ namespace ObjCRuntime {
 			return ObjectWrapper.Convert (GetNSObject (ptr, type, MissingCtorResolution.ThrowConstructor1NotFound, true, out created));
 		}
 
-		static void Dispose (IntPtr mobj)
+		static void Dispose (IntPtr gchandle)
 		{
-			((IDisposable) ObjectWrapper.Convert (mobj)).Dispose ();
+			((IDisposable) GetGCHandleTarget (gchandle)).Dispose ();
 		}
 
 		static bool IsParameterTransient (IntPtr info, int parameter)