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
https://golang.org/cl/5633050