MFC 265861, 265870:

  Make the hardware memory and instruction barrier functions work on armv4
  and armv5 as well.

  Add cpu_l2cache_drain_writebuf(), use it to implement generic_bs_barrier().
MFC 265913, 265914:

  Interrupts need to be disabled on entry to cpu_sleep() for ARM.  Given
  that and the need to be in a critical section when switching to idleclock
  mode for event timers, use spinlock_enter()/exit() to achieve both needs.

  Clean up some style nits.