configure: enable PIC on s390(x)

The s390 architecture requires shared libraries to be built in PIC mode.
Otherwise applications will get wrong relocations at run-time, leading
to confusing segmentation faults.

CC: libav-stable@libav.org

configure

@@ -3220,6 +3220,10 @@ case "$arch" in
         check_64bit ppc ppc64 'sizeof(void *) > 4'
         spic=$shared
     ;;
+    s390)
+        check_64bit s390 s390x 'sizeof(void *) > 4'
+        spic=$shared
+    ;;
     sparc)
         check_64bit sparc sparc64 'sizeof(void *) > 4'
         spic=$shared