cpu: Useful getters for ActivityRecorder
cpu: Useful getters for ActivityRecorder

Add some useful getters to ActivityRecorder
(transplanted from 0f00b9e7305a23f36a703520beaf78e81147911c)

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