Add __cxa_deleted_virtual to libcxxrt's version map.

This symbol can sometimes be emitted by clang++, and was not yet
exported from libcxxrt. Attempt to be compatible with libsupc++ by
using the same CXXABI_1.3.6 symbol version.

Reported by: yuri@rawbw.com
PR: 200863
Reviewed by: emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D2850