[IA64] - Allow IPIs in timer loop

Allow pending IPIs to interrupt a timer interrupt that is looping
in the do_timer() "while" loop in timer_interrupt(). (Interrupts are
allowed at only 1 spot in the code).

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>