mem-cache: Forward snoops when the cache is not responding

mem-cache: Forward snoops when the cache is not responding

When the MSHR is handling a request that will make the block dirty the
current cache commits respond. When that's not the case the cache
should forward any snoops. This CL fixes MSHR::handleSnoop() to
implement this behavior.

Change-Id: I207e3ca4968fd9528fd4cdbfb3eb95f470b4744d
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23668
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>