base: Add a new type of CallbackQueue.

base: Add a new type of CallbackQueue.

This type is templated on what arguments the callbacks in it accept, and
it inherits directly from std::list instead of containing one and
forwarding selected members.

This version is called CallbackQueue2, but once all CallbackQueue
instances have been replaced it will be renamed to CallbackQueue.

Issue-on: https://gem5.atlassian.net/browse/GEM5-698
Change-Id: I32ab7454ea8c6a2af31cbcf5d4932a069ace1cb5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32642
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>