oleaut32: Dynamic invocation for x86

[Juan Lang]

Thanks for the review, Henri.

> I don't know if there's a reason the general approach couldn't work,
> but the code in the patch is gcc specific, so I think it should at
> least be protected by __GNUC__ in addition to __i386__.

We don't protect by __GNUC__ for any of our other assembly either,
even though it all uses GNU assembly syntax.  So personally I think
that part's okay.
--Juan