base-filesystem: check for __s390x__ first

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."
      |           ^~~~~~~