Is this fixed?   I similarly hosed my system last night; it booted and
stopped after attaching ubt0.   I was able to power off the adaptor
with fn-f5 and a detach message was printed.  ps via ddb showed system
threads only.  Booting '-as' with /rescue/init worked.  Then running
ktrace on /bin/sh core dumped, and there was nothing useful in the
trace.

Does going back to libc 1.127 fix things?