sim: Add a listener checker to probes

sim: Add a listener checker to probes

Add a function to check if a probe has listeners. This can be
used to avoid performing costly tasks when no one is listening.

Change-Id: I8996a0ea298cb7cf97ac8aa9e627331a22bea26e
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38175
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>