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>

configure

@@ -728,7 +728,7 @@ ARCH=
 # Normalise host CPU name and set ARCH.
 # Note that this case should only have supported host CPUs, not guests.
 case "$cpu" in
-  ppc|ppc64|s390|s390x|sparc64|x32|riscv32|riscv64)
+  ppc|ppc64|s390x|sparc64|x32|riscv32|riscv64)
     supported_cpu="yes"
   ;;
   ppc64le)