For the first time in my experience, the Raspberry Pi3 has been able to self-host a full system build for me. I had to knudge it along at one point, but it eventually completed a full buildworld and buildkernel.
On a lark, I changed /etc/fstab to bump the tmpfs size from 50M to 80M, rebooted, started buildkernel again, and buildkernel completed. The installs worked, and the Pi3 successfully booted with the new system.
makewith 3 threads didn't work (buildworld stopped). So I dropped the number of threads to 1, and buildworld completed. But buildkernel stopped with some out of space condition:
linking kernel.full ctfmerge -L VERSION -g -o kernel.full ... objcopy: elf_update() failed: I/O error: No space left on device *** [kernel.full] Error code 1