cpu: Idle CPU status logic revised
cpu: Idle CPU status logic revised

This patch sets the CPU status to idle when the last active thread gets
suspended.
(transplanted from 94901e131a7f2452b3cf2b2740be866b4a6f404d)

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