mem-cache: Use CircularQueue for the STeMS's RMOB

mem-cache: Use CircularQueue for the STeMS's RMOB

Replace rmobHead functionality with a CircularQueue. As a side
effect, RMOB entry's valid bit was removed.

Change-Id: If0b39cfe15de1e47dde0689a0ebc46b623ec1e55
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24539
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Javier Bueno Hedo <javier.bueno@metempsy.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>