s390x: test if the DXC is correctly stored

This was not properly handled in TCG for all DATA exceptions. Let's
use a simple example.

CRT produced a trap via a DATA exception with DXC 0xff, when in this
case both registers are equal (which is the case because we are using
the same register).

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180824115059.1517-5-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>