[release-branch.go1.2] runtime: fix data race in GC

««« CL 52090045 / 302bdb5b08b1
runtime: fix data race in GC
Fixes #5139.
Update #7065.

R=golang-codereviews, bradfitz, minux.ma
CC=golang-codereviews
https://golang.org/cl/52090045
»»»

LGTM=r
R=golang-codereviews, r
CC=golang-dev
https://golang.org/cl/69790043