x86: change NR_CPUS arrays in smpboot_64

Change the following static arrays sized by NR_CPUS to
per_cpu data variables:

	task_struct *idle_thread_array[NR_CPUS];

This is only done if CONFIG_HOTPLUG_CPU is defined
as otherwise, the array is removed after initialization
anyways.

Signed-off-by: Mike Travis <travis@sgi.com>
Reviewed-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>