tcg-s390: correctly detect s390 with a 64-bit kernel

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

configure

@@ -193,6 +193,12 @@ elif check_define __mips__ ; then
   cpu="mips"
 elif check_define __ia64__ ; then
   cpu="ia64"
+elif check_define __s390__ ; then
+  if check_define __s390x__ ; then
+    cpu="s390x"
+  else
+    cpu="s390"
+  fi
 else
   cpu=`uname -m`
 fi