minor changes to the C semantics to add support for native calls

@chathhorn please review.

This code is not in its final form; I still need to deal with some issues surrounding linking. But I feel it's better to proceed incrementally by means of doing individual case studies of executing programs. I have tested this code on several functions in GLIBC that take no parameters and return basic types and it seems to work.
Sorry, I meant this.

Some of the older stuff uses that dash-separated style and I haven't been too proactive about updating it. Productions with an initial capital have generally been intended to represent the "concrete" syntax (the input to the semantics, the output of the CIL parser).