x86: increase MAX_APICS for very large x86-64 configs

Increase the maximum number of apics when running very large
configurations. This patch has no affect on most systems.

The patch has no effect on any 32-bit kernel. It adds ~4k to the size
of 64-bit kernels but only if NR_CPUS > 255.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>