The "irq_vector[]" array is indexed by the sum of all RTEs in all I/O
APICs, and is not necessarily limited by the x86 CPU irq vector inputs.

In fact, the irq vector index would overflow on big machines with lots
of IO APIC's, causing the boot to fail.

So grow the array for the big SMP boxes, keeping the default the same as
before (and shrink the vector entry size down to a 8-bit value, since
that's the size of the actual CPU vector entry).