[IA64] remove staled comments in asm/system.h

# define switch_to(prev,next,last)	__switch_to(prev, next, last)

#endif

/*

 * On IA-64, we don't want to hold the runqueue's lock during the low-level context-switch,

 * because that could cause a deadlock.  Here is an example by Erich Focht:

 *

 * Example:

 * CPU#0:

 * schedule()

 *    -> spin_lock_irq(&rq->lock)

 *    -> context_switch()

 *       -> wrap_mmu_context()

 *          -> read_lock(&tasklist_lock)

 *

 * CPU#1:

 * sys_wait4() or release_task() or forget_original_parent()

 *    -> write_lock(&tasklist_lock)

 *    -> do_notify_parent()

 *       -> wake_up_parent()

 *          -> try_to_wake_up()

 *             -> spin_lock_irq(&parent_rq->lock)

 *

 * If the parent's rq happens to be on CPU#0, we'll wait for the rq->lock

 * of that CPU which will not be released, because there we wait for the

 * tasklist_lock to become available.

 */

#define __ARCH_WANT_UNLOCKED_CTXSW

#define ARCH_HAS_PREFETCH_SWITCH_STACK

#define ia64_platform_is(x) (strcmp(x, platform_name) == 0)