mem-cache: Factor out CPack's dictionary functionality

mem-cache: Factor out CPack's dictionary functionality

Factor out dictionary functionality of CPack, so that it can be
used easily for other compressors.

As a side effect, create an addToDictionary function to allow
subclasses to chose how to handle insertion.

Change-Id: I02fae4e98b02db5a40467ec470b71020d5e867cb
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21147
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>