Updated ioapic, BTX error fix

Sepherosa Ziehau’s recent commit changing how ioapic works may help anyone who has previously had trouble compiling a multiprocessor kernel with IO_APIC enabled.  Try it, if that applies to you.

Also, Jordan Gordeev has a potential fix for anyone who has had a failed boot with a ‘BTX Halted’ message; you will have to retrieve it from his Git repo.