runtime: disable preemption during runtime.settype
runtime: disable preemption during runtime.settype

It assumes that the m will not change, and the m may
change if the goroutine is preempted.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/11560043

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