runtime: fix typo in ARM code
runtime: fix typo in ARM code

The typo was introduced by one of Dmitriy's CLs this morning.
The fix makes the ARM build compile again; it still won't pass
its tests, but one thing at a time.

TBR=dvyukov
CC=golang-codereviews
https://codereview.appspot.com/55770044

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