Fix heap profiling times
Now that the heap census runs in the middle of garbage collections, the "CPU time" it was calculating included any CPU time used so far in the current GC. This could cause CPU time to appear to go down, which means hp2ps complained about "samples out of sequence". I'm not sure if this is the nicest way to solve this (maybe resurrecting mut_user_time_during_GC would be better?) but it gets things working again.
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