Note that without CONFIG_HOTPLUG_CPU, online cpus == possible cpus, so
substitutions are a noop.

- Changes show_stat to print out stats for every possible cpu, not
  just online CPUs.

- Allocate mem in stat_open on possible, not online_cpus.

- Add conventient macros to cpu.h: especially cpu_is_offline() for
  testing if a cpu is still online.

- Add a num_possible_cpus() similar to num_online_cpus(), and define
  cpu_possible_mask for UP.

- Allow printk on down cpus once system is running.

- Mask cpumask with possible, not online cpus, for sys_getaffinity().