Time: Add a mechanism to prevent M5 from running faster than real time.

M5 skips over any simulated time where it doesn't have any work to do. When
the simulation is active, the time skipped is short and the work done at any
point in time is relatively substantial. If the time between events is long
and/or the work to do at each event is small, it's possible for simulated time
to pass faster than real time. When running a benchmark that can be good
because it means the simulation will finish sooner in real time. When
interacting with the real world through, for instance, a serial terminal or
bridge to a real network, this can be a problem. Human or network response time
could be greatly exagerated from the perspective of the simulation and make
simulated events happen "too soon" from an external perspective.

This change adds the capability to force the simulation to run no faster than
real time. It does so by scheduling a periodic event that checks to see if
its simulated period is shorter than its real period. If it is, it stalls the
simulation until they're equal. This is called time syncing.

A future change could add pseudo instructions which turn time syncing on and
off from within the simulation. That would allow time syncing to be used for
the interactive parts of a session but then turned off when running a
benchmark using the m5 utility program inside a script. Time syncing would
probably not happen anyway while running a benchmark because there would be
plenty of work for M5 to do, but the event overhead could be avoided.

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