test-execute: check for s390x first and duplicate test

s390x will define both s390x and s390, so exec-personality-s390.service is ran
in both cases but fails on s390x, as the personality returned is s390x.
Split the test and check specifically for s390x.

test/test-execute/exec-personality-s390x.service

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Description=Test for Personality=s390x
+
+[Service]
+ExecStart=sh -x -c 'c=$$(uname -m); test "$$c" = "s390x"'
+Type=oneshot
+Personality=s390x