cpu: Create a PCEventScope class to abstract the scope of PCEvents.

cpu: Create a PCEventScope class to abstract the scope of PCEvents.

This abstraction will allow scheduling PCEvents for a particular
ThreadContext, all contexts on a CPU, all contexts in a system, etc.,
and delegates scheduling and removing events to each particular scope.

Right now the PCEventQueue is the only implementor of the PCEventSCope
interface.

Change-Id: I8fb62931511136229915c2e19d36aae7ffdec9df
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22099
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>