cpu: Only iterate over possible threads on the o3 cpu
cpu: Only iterate over possible threads on the o3 cpu

Some places in O3 always iterated over "Impl::MaxThreads" even if a CPU had
fewer threads.  This removes a few of those instances.
(transplanted from c870b43d2ba62783e056076522b3d15f278cc9a8)

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin