runtime: fix data race in GC Fixes issue 5139. Update issue 7065.
runtime: fix data race in GC
Fixes issue 5139.
Update issue 7065.

R=golang-codereviews, bradfitz, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/52090045

firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old