x86: fix mpparse fallout

UP builds with LOCAL_APIC=y and IO_APIC=n fail with a missing
reference to mp_bus_not_pci. Distangle the mpparse code some more and
move the ioapic specific bus check into a separate function.

This code needs sume urgent un#ifdef surgery all over the place.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>