runtime: fix cgocallback stack size on amd64p32.
runtime: fix cgocallback stack size on amd64p32.

LGTM=dave
R=rsc, dave, iant
CC=golang-codereviews
https://codereview.appspot.com/73160043

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