runtime: revert 3c2cddfbdaec
runtime: revert 3c2cddfbdaec

It appears, syscall.NewCallback still
uses heap to store executable code.

R=golang-dev, khr
CC=golang-dev
https://codereview.appspot.com/9060046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang