sim: Add a new gem5 op for workload events.

sim: Add a new gem5 op for workload events.

This is a way to send a very generic poke to the workload so it can do
something. It's up to the workload to know what information to look for
to interpret an event, such as what PC it came from, what register
values are, or the context of the workload itself (is this SE mode? which
OS is running?).

Change-Id: Ifa4bdf3b5c5a934338c50600747d0b65f4b5eb2b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34162
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>