runtime: revert change to PoisonPtr value
runtime: revert change to PoisonPtr value

Submitted accidentally in CL 83630044.
Fixes various builds.

TBR=khr
CC=golang-codereviews
https://codereview.appspot.com/83100047

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