syscall: NetBSD/ARM support

R=golang-dev, rsc, dave
CC=golang-dev
https://codereview.appspot.com/7288050
sync/atomic: support NetBSD/ARM (ARM11 or above)

R=golang-dev, rsc, dave
CC=golang-dev
https://codereview.appspot.com/7287044
go/build, runtime/cgo: cgo support for NetBSD/ARM

R=golang-dev, rsc, dave
CC=golang-dev
https://codereview.appspot.com/7229082