Port the NetBSD ubsan runtime to the FreeBSD kernel.

This allows us to build the ubsan code added in r340189 into the kernel
with the KUBSAN option. This will report when undefined behaviour is
detected in the currently running kernel.

As it can be large, the kernel is 65MB on arm64, loader may not be able to
load the kernel on all architectures so is disabled by default for now.

Sponsored by: DARPA, AFRL