There’s been a number of people reporting various breakages during install or boot time. To weed out problems caused by old data right away, remember to:
rm -rf /usr/obj
make buildworld
make buildkernel
make installkernel
make installworld
mergemaster
reboot
…and it might be good idea to add a step following:
rm -rf /usr/obj
make buildworld
make buildkernel
make installkernel
rm -rf /usr/include/* <– Add this step.
make installworld
mergemaster
reboot