Zero: Support AsyncGetCallTrace

This is a Zero infrastructure improvement that makes Zero VM work with AsyncGetCallTrace, and by extension, async-profiler.

Zero is quite odd in stack management. The "real" stack actually contains the C++ Interpreter and the rest of VM code. The Java stack is reported through the usual "frame" mechanism the rest of VM uses to get the mapping from Template Interpreter, stub, and compiled code. So, to support Java-centric AsyncGetCallTrace, we t "only" need Zero to report the proper Java frames from its ZeroStack from the profiling/signal handlers.