target/s390x: define TCG_GUEST_DEFAULT_MO for MTTCG

target/s390x: define TCG_GUEST_DEFAULT_MO for MTTCG

MTTCG should be enabled by default whenever the memory model allows
it. s390x was missing its definition of TCG_GUEST_DEFAULT_MO meaning
the user had to manually specify  --accel tcg,thread=multi.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: David Hildenbrand <david@redhat.com>
Message-Id: <20190118171848.27332-1-alex.bennee@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>