cpu: Don't print the CPU name when a (Break|Panic)PCEvent happens.

cpu: Don't print the CPU name when a (Break|Panic)PCEvent happens.

This requires reaching into the threadcontext to access the CPU
pointer, and also isn't all that useful since it's more important what
event happened, not what CPU happened to be running the code at that
time.

Change-Id: I368707c804dff9bd349f3261bdcd08be55c5d04a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22103
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>