Revert "Unwind past an interrupt handler correctly on arm or at pc==0"

The aarcht64-ubuntu bot is showing a test failure in TestHandleAbort.py
with this patch.  Adding some logging to that file, it looks like
the saved register context above the trap handler does not have
save state for $pc, but it does have it for $lr on that platform.
I need to fall back to looking for $lr if the $pc cannot be retrieved.
I'll update the patch and re-commit once that's fixed.

This reverts commit edc4f4c9.