arch,sim,misc: Add a new m5 op "sum" which just sums its inputs.

arch,sim,misc: Add a new m5 op "sum" which just sums its inputs.

This very simple and mostly useless operation has no side effects, and
can be used to verify that arguments are making it into gem5, being
operated on, and then that a result can be returned into the simulation.

Change-Id: I29bce824078526ff77513c80365f8fad88fef128
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27557
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>