misc: Make the stats callbacks use CallbackQueue2.

misc: Make the stats callbacks use CallbackQueue2.

Issue-on: https://gem5.atlassian.net/browse/GEM5-698
Change-Id: Idcbe04bdf4299925f321aa0ece263d86ed3fc8df
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32645
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>