configure: fix S390X CPUNAME:

The value was changed to S390x, presumably by accident, breaking the
build (regression from 480b32d3).

Change-Id: Icca90818c6363855437a273714a8317becdfd0e5

configure.in

@@ -3459,7 +3459,7 @@ linux-gnu*)
         ;;
     s390x)
         CPU=3
-        CPUNAME=S390x
+        CPUNAME=S390X
         RTL_ARCH=S390x
         LIB64="lib64"
         OUTPATH=unxlngs390x