mem-cache: Make compression size threshold a percentage

mem-cache: Make compression size threshold a percentage

By changing the parameter into a percentage, changing the block
size will automatically reconfigure the size threshold. Also,
change the default percentage to 50% to avoid storing blocks
unlikely to co-allocate in compressed format.

Change-Id: I1458f19db39becc2d40c00269132fea01770016f
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33385
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>