x86: put timer_rand_state pointer into irq_desc

      irq_timer_state[] is a NR_IRQS sized array that is a side-by array to
the real irq_desc[] array.

Integrate that field into the (now dynamic) irq_desc dynamic array and
save some RAM.

v2: keep the old way to support arch not support irq_desc

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>