runtime: fix netbsd/arm build
Currently fails with:
fatal error: runtime: stack split during syscall
goroutine 2 [stack split]:
_vasop(0x3ac4a0, 0x505f8f00, 0x7a5a8, 0x7, 0x1ed3797f, ...)
        src/pkg/runtime/vlrt_arm.c:513 fp=0x505f8ecc
runtime.semasleep(0xf8475800, 0xd)
        src/pkg/runtime/os_netbsd.c:97 +0x178 fp=0x505f8efc

R=rsc
CC=golang-dev
https://codereview.appspot.com/12246043