runtime: collect profiles even while on g0 stack
runtime: collect profiles even while on g0 stack

Fixes issue 6417

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/14231047

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