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

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang