[ObjCRuntime] Work around a difference in .NET where Type.IsInterface doesn't work the same way for byref types.

[rolfbjarne]

Ref: dotnet/runtime#39068

Fixes this test:

MonoTouchFixtures.ObjCRuntime.RegistrarTest
    [FAIL] RefOutTest_INSCoding : System.InvalidCastException : Unable to cast object of type 'Foundation.NSString' to type 'Foundation.INSCoding&'.
        at ObjCRuntime.Runtime.GetINativeObject(IntPtr ptr, Boolean owns, Type target_type, Type implementation)
        at ObjCRuntime.Runtime.GetINativeObject(IntPtr ptr, Boolean owns, Type target_type)
        at ObjCRuntime.Runtime.GetINativeObject_Dynamic(IntPtr ptr, Boolean owns, IntPtr type_ptr)
        at ObjCRuntime.Runtime.get_inative_object_dynamic(IntPtr obj, Boolean owns, IntPtr type, Int32& exception_gchandle)
        --- End of stack trace from previous location ---
        at MonoTouchFixtures.ObjCRuntime.RegistrarTest.RefOutTest_INSCoding() in /Users/rolf/work/maccore/squashed-onedotnet/xamarin-macios/tests/monotouch-test/ObjCRuntime/RegistrarTest.cs:line 2908
        at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

[monojenkins]

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