[DotGNU]callvirt change to cscc

[Rhys Weatherley]

I just changed cscc and ilrun to use "callvirt" instructions for
non-virtual instance methods, as all the other C# compilers do that.
(Personally, I think the previous behaviour was better).

This change will cause calls on instance methods to always throw
NullReferenceException when null is supplied as "this", even if
the method isn't virtual.

It was a simple change, but let me know if this breaks anything.

Cheers,

Rhys.