See https://bugs.freebsd.org/200863 for details. The patch there adds
it to the first part of libcxxrt's version map, under CXXABI_1.3, but
when I checked upstream libsupc++, they put this symbol under
CXXABI_1.3.6. So we should probably also put it under this version.
That said, I'm not entirely sure about the dependencies, e.g. whether
CXXABI_1.3.6 shoudl be depending on CXXABI_1.3.1 or something else.
Alternatively, we could put these under the CXXRT_1.0 part, since some
of the functions there (e.g cxa_allocate_dependent_exception and
cxa_free_dependent_exception) are in CXXABI_1.3.6 in libsupc++. So we
are not compatible with libsupc++ on that front anyway.