runtime: fix arm build

The current failure is:
fatal error: runtime: stack split during syscall
goroutine 2 [stack split]:
_addv(0xb6fa0f28, 0xd0a5112e, 0x13156d6e, 0xf8475800, 0xd)
        /usr/local/go/src/pkg/runtime/vlrt_arm.c:66 fp=0xb6fa0ef8
notetsleep(0xb6fa0f9c, 0xf8475800, 0xd, 0x0, 0x0, ...)
        /usr/local/go/src/pkg/runtime/lock_futex.c:156 +0xd0 fp=0xb6fa0f18
runtime.notetsleepg(0xb6fa0f9c, 0xf8475800, 0xd)
        /usr/local/go/src/pkg/runtime/lock_futex.c:197 +0x74 fp=0xb6fa0f3c

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12052043