fastmodel: Implement PC based events.

fastmodel: Implement PC based events.

These use the IRIS breakpoint API to stop the models at the appropriate
points. There seems to be a slightly wonky interaction between
breakpoints and stepping, where if you stop at a breakpoint and then
step, you might end up moving forward more than the number of requested
instructions.

Change-Id: I31f13a120cfc1ad2ec3669ee8befd6d21b328bb2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22122
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>