calling dll native entry point for mixed assemblies

[Vincent Povirk]

> So then I will split this patch into two or three, look couple of times
> more and send it.

It seemed like one logical change to me, I don't think it needs
splitting. (Except for the change to FixupVTable which seems
unrelated.)

How can I build a DLL to test this? The default managed c++ dll
doesn't seem to include an entry point.