runtime: fix GC bitmap corruption Fixes issue 8299.
runtime: fix GC bitmap corruption
Fixes issue 8299.

R=golang-codereviews
CC=golang-codereviews, khr, rsc
https://codereview.appspot.com/103640044

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