Fix build failure on NetBSD/arm.

patch-ah (new):
Fix wrong placed #else clause against #if defined(__linux__) in cpu-arm.c.

patch-ba:
Check NetBSD's native atomic_ops in configure even in arm case.
configure checks $host_cpu first, not $host_os on checks for
native atomic operations.
(In upstream it seems fixed to see $host_os first but
  it might still have problem around NetBSD version checks)

(tsutsui)