undone because the change slows down profile collection
significantly and unpredictable at times (see comments
at https://golang.org/cl/14231047 for details)

««« original CL description
runtime: collect profiles even while on g0 stack

Fixes #6417

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/14231047
»»»

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/14535046