configure: Remove s390 (31-bit mode) from the list of supported CPUs

configure: Remove s390 (31-bit mode) from the list of supported CPUs

On IBM Z, KVM in the kernel is only implemented for 64-bit mode, and
with regards to TCG, we also only support 64-bit host CPUs (see the
check at the beginning of tcg/s390/tcg-target.inc.c), so we should
remove s390 (without "x", i.e. the old 31-bit mode CPUs) from the
list of supported CPUs.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190928190334.6897-1-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>