base-filesystem: check for __s390x__ first

On s390x both __s390__ and __s390x__ are defined, and with the original
order we'd go through the __s390__ branch and emit a warning:

[169/2118] Compiling C object src/shared/libsystemd-shared-256.a.p/base-filesystem.c.o
../src/shared/base-filesystem.c:136:11: note: ‘#pragma message: Please add an entry above specifying whether your architecture uses /lib64/, /lib32/, or no such links.’
  136 | #  pragma message "Please add an entry above specifying whether your architecture uses /lib64/, /lib32/, or no such links."
      |           ^~~~~~~

src/shared/base-filesystem.c

@@ -120,13 +120,13 @@ static const BaseFilesystem table[] = {
 #  else
 #    error "Unknown RISC-V ABI"
 #  endif
-#elif defined(__s390__)
-        /* s390-linux-gnu */
 #elif defined(__s390x__)
         { "lib64",    0, "usr/lib/"LIB_ARCH_TUPLE"\0"
                          "usr/lib64\0"
                          "usr/lib\0",                "ld-lsb-s390x.so.3" },
 #    define KNOW_LIB64_DIRS 1
+#elif defined(__s390__)
+        /* s390-linux-gnu */
 #elif defined(__sparc__)
 #endif
         /* gcc doesn't allow pragma to be used within constructs, hence log about this separately below */