bib should not generate gce image on s390x machine

[chunfuwen]

'''
Since in Google cloud, it doesn't support s390x guest. Therefore it make sense it remind us that bib can not generate gce disk image format on s390x machine

sudo podman run --rm -it --privileged --pull=newer --security-opt label=type:unconfined_t -v /var/lib/libvirt/images/output:/output -v /var/lib/libvirt/images/config.json:/config.json -v /var/lib/libvirt/images/auth.json:/run/containers/0/auth.json quay.io/centos-bootc/bootc-image-builder:latest --type gce --tls-verify=false --config /config.json quay.io/centos-bootc/centos-bootc:stream9

...
manifest - finished successfully
build: ac3f8e47f57148704d7a39bfe769dfa72b8234cf58d8199c295530c637f70418
image: b68b0fa35fe06168760239bbd635d8e1da49c212a36079b8bb084e189c3dae4d
qcow2: ca72e9940f3dbc765b446272dd19e8fafdd053a890e95f3aa3bdf2e733cfd08b
vmdk: 50268525723829699c414b5eca353fa7019d9144886d7ef2e8b543d54dde4e7f
vpc: cd14aec7b55fa2e2a14b1b2820846d81164e22f16540a1b7c1b31b582fc8cf40
ovf: c167e544e9334d8542d4b12624e074c4261b94a6f22cc34a154193218aae3d4a
archive: 0f77fb7c439a207af66ce14aa88f8d7c7de6dd56314ba06239a1b6e1b02f41d2
gce: 9bc225ce35a036efbf8b60049b547aafe960e17434eee4bf0122cfc7696e16b8
Build complete!
Results saved in
'''