cpu: Make the ThreadContext a PCEventScope.

cpu: Make the ThreadContext a PCEventScope.

Both the thread and system's PCEventQueue are checked when appropriate.

Change-Id: I16c371339c91a37b5641860d974e546a30e23e13
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22105
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>