To workaround an errata on certain Pentium Pro CPUs, i386 disables
the local APIC in initializecpu() and re-enables it if the APIC code
decides to use the local APIC after all. Rework this workaround
slightly so that initializecpu() won't re-disable the local APIC if
it is called after the APIC code re-enables the local APIC.