panic: unregistered use of FPU in kernel

[Alan Somers]

The 12.1-PRERELEASE (FreeBSD-12.1-PRERELEASE-amd64-20190913-r352266.qcow2)
VM image instapanics with the error message:

panic: Unregistered use of FPU in kernel
stack trace:
...
ffs_ckhash_ch
bufdone
g_io_deliver
g_io_deliver
g_io_deliver
g_disk_done
vtblk_vq_intr
ithread_loop
fork_exit
...

I see a similar panic on HEAD snapshots, reproducible as far back as the
June-7 snapshot (the oldest one on the ftp server).  I'll continue
investigating.
-Alan