Patch from James Cleverdon & John Stultz

The IRQ balancing code currently assumes that the logical apicid is
always '1 << cpu', which is not true for the larger platforms.
We express this as an abstracted macro instead, and move the
cpu_to_logical_apicid definition to subarch, so we can make it exactly
"1 << cpu" for normal machines - maximum speed, minimum change risk.

A couple of things are abstracted from the smp_boot_cpus loop in order
to enable us to use the bios_cpu_apicid array to boot cpus from without
disturbing the code path of current machines.