Improve the precision on variable arguments function

[arthaud]

The current version of IKOS does not handle variable arguments precisely.

The previous version was very precise, but at a big complexity cost: the analyzer was basically simulating very low level details, such as pushing the arguments of a function call to a var-arg function on a symbolic stack array. It was also dependent on the architecture.

I did not want to investigate time implementing this again when I designed the new abstract representation. I believe variable arguments functions are pretty rare in embedded systems, and should be avoided. Also, they are mostly "leaves", so they won't propagate warnings all over the place.

In the future, we should think about a better way to handle these. This is low-priority though. The current version is sound regarding var-arg functions.