Disable the smallest UMA bucket size on 32-bit platforms.

With r357314, sizeof(struct uma_bucket) grew to 16 bytes on 32-bit
platforms, so BUCKET_SIZE(4) is 0. This resulted in the creation of a
bucket zone for buckets with zero capacity. A more general fix is
planned, but for now this bandaid allows 32-bit platforms to boot again.

PR: 243837
Discussed with: jeff
Reported by: pho, Jenkins via lwhsu
Tested by: pho
Sponsored by: The FreeBSD Foundation