cpu: Allow setWhen on trace objects
cpu: Allow setWhen on trace objects

Allow setting of 'when' in trace records.  This allows later times
than the arbitrary record creation point to be used as inst. times
(transplanted from 590ba88231633351012964bfcf01e1166d9de8cc)

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5