sims/external/gem5: properly speed up simulations involving checkpointing

For SimBricks messages sent by gem5, we want the simulation to look like it started at tick 0. The reason is that otherwise, after restoring a checkpoint, connected simulators first have to catch up, i.e. simulate until the checkpoint tick before the simulation can continue.

gem5 @ 2c500a6a

-Subproject commit fc854791fc642370aef3ad3d4b87d2395d7ea1e3
+Subproject commit 2c500a6a7527a1305e1a8e03f53ea11e90b71b73