runtime: use GOTRACEBACK to decide whether to show runtime frames
runtime: use GOTRACEBACK to decide whether to show runtime frames

Right now, GOTRACEBACK=0 means do not show any stack traces.
Unset means the default behavior (declutter by hiding runtime routines).

This CL makes GOTRACEBACK=2 mean include the runtime routines.
It avoids having to recompile the runtime when you want to see
the runtime in the tracebacks.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5633050

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