dmd Arm64 - first function compile!

[Richard (Rikki) Andrew Cattermole]

Awesome work!

You are inspiring me to consider doing a bit of work on the backend.

I've already checked, first possible thing to consider is to move all 
the object format stuff from frontend into backend to make it more self 
contained.

Who knows, maybe that backend could be attractive in the hobbyist 
compiler community yet!