mem-cache: TempCacheBlk allocates and destroys its own data

mem-cache: TempCacheBlk allocates and destroys its own data

This change is because I want to make CacheBlk::data private, so that
I can track all the places which write to it. But to keep that commit
smaller (it is pretty big, because of all the places which might
change it), I have split this into a commit of its own.

Change-Id: I15a2fc1752085ff3681f5c74ec90be3828a559ea
Reviewed-on: https://gem5-review.googlesource.com/11829
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>