withTimings reports inaccurate times with `-j`

The time information in withTimings is computed using getCPUTime which counts cpu time on a process level, when there are multiple threads this leads to inaccurate results as withTiming is supposed to compute how long something takes to run on a specific thread.

It would be useful to be able to track per Haskell-thread time for instrumentation like this or at the very least change the counter to use something else like the real time. At the moment the timing information is very misleading with -jN.