sim, stats: Move global stats to Root

sim, stats: Move global stats to Root

Global stats are currently exposed using the legacy stat system (i.e.,
without a parent group). This change moves global stats from
stat_control.cc to a group that gets exported from the Root object.

The implementation adds the Root::Stats class which has a single
global instance. This instance is exposed to the rest of the simulator
using the global rootStats symbol. The intention is that objects that
need global statistics in formulas access them through the rootStats
object.

The global names simSeconds, simTicks, simFreq, and hostSeconds are
now references to their respective members in the rootStats object.

Change-Id: I267b5244a0bcca93dd2dcf03388e7085bdd79c9e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35616
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>