Pull the turbo, use a drill press to get the bolt bolt out, then Heli-coil the hole.
This is an easy fix that you will only make more difficult by trying to half ass the repair. Dremel the head and use a screwdriver? Good luck - it was stuck bad enough that you snapped the bolt... unless you think you can turn a screwdriver harder than the force needed to snap the bolt, this isn't gonna work...