cpu: Timebuf const accessors
cpu: Timebuf const accessors

Add const accessors for timebuf elements.
(transplanted from 1ab8753de4d86d0e6f2e9c353b94abd068d0cef2)

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