runtime: handle gdb breakpoint in x86 traceback
runtime: handle gdb breakpoint in x86 traceback

This lets stack splits work correctly when running under gdb
when gdb has inserted a breakpoint somewhere on the call
stack.

Fixes issue 6834.

R=golang-codereviews, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/48650043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang