When an exception is thrown the unwinder must unwind its own C source
(starting with _Unwind_RaiseException in UnwindLevel1.c), so it needs to
be built with unwinding data.
They are not yet connected to the build, but I am adding them to allow
for easier testing, ports exp-runs, etc.
Reviewed by: ed
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8188