mem-cache: Add an uncompressed pattern to compressors

mem-cache: Add an uncompressed pattern to compressors

The uncompressed pattern always stores the original data, and therefore
it is always successful. All of the derived classes of the dictionary
compressor must have this pattern as the last pattern of the pattern
factory.

Change-Id: I2a38fd56630d88ef8b918220dc4c2824a196a8a2
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21149
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>