So here's the partial conclusion:
All toolchain/kernels I've built with OpenBSD 6.4 and clang 6.0 trigger this panic. This is on amd64 and arm64.
I built a Linux VM on my OpenBSD/amd64 box, and...
All toolchain/kernels I've built with Linux amd64 and gcc 6.3 do not trigger this panic.
This is on the same set of sources (shared via NFS), config files and mk.conf.
I've had issues building the toolchain on OpenBSD using gcc (either the built-in 4.2.1 or 4.9.4 from ports), so I can only guess that it's an OpenBSD rather than clang quirk.
I'll try to investigate some more at a later date.
Many thanks for the help and pointers which helped me work it out.