runtime: print g0 stack if we throw on it and GOTRACEBACK>=2.
runtime: print g0 stack if we throw on it and GOTRACEBACK>=2.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/11385045

apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme