ia64: NR_CPUS and number of CPUs

While building a kernel for our 4-way Lion box, I made the
mistake of setting NR_CPUS to 4.  Little did I know that the
Lion ACPI tables always list 8 CPUs (with only the first N
enabled), and so the resulting kernel overflowed the
smp_boot_data.cpu_phys_id array, crashed and burned.