hrtimer: Prevent stale expiry time in hrtimer_interrupt()

hrtimer_interrupt() has the following subtle issue:

hrtimer_interrupt()
  lock(cpu_base);
  expires_next = KTIME_MAX;

  expire_timers(CLOCK_MONOTONIC);
  expires = get_next_timer(CLOCK_MONOTONIC);
  if (expires < expires_next)
    expires_next = expires;

  expire_timers(CLOCK_REALTIME);
    unlock(cpu_base);
    wakeup()
    hrtimer_start(CLOCK_MONOTONIC, newtimer);
    lock(cpu_base();  
  expires = get_next_timer(CLOCK_REALTIME);
  if (expires < expires_next)
    expires_next = expires;

So because we already evaluated the next expiring timer of
CLOCK_MONOTONIC we ignore that the expiry time of newtimer might be
earlier than the overall next expiry time in hrtimer_interrupt().

To solve this, remove the caching of the next expiry value from
hrtimer_interrupt() and reevaluate all active clock bases for the next
expiry value. To avoid another code duplication, create a shared
evaluation function and use it for hrtimer_get_next_event(),
hrtimer_f...