mem-cache: Protect tag from being mishandled

mem-cache: Protect tag from being mishandled

Make the tag variable private, so that every access to it must pass
through a getter and a setter. This protects it from being incorrectly
updated when there is no direct match between a tag and a data entry,
as is the case of sector, compressed, decoupled, and many other table
layouts.

As a side effect, a block matching function has been created, which
also depends directly on the mapping between tag and data entries.

Change-Id: I848a154404feb5cbcea8d0fd2509bf49e1d73bd0
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34955
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>