C++ ports using virtual function deletion (c++11 feature) are broken when clang is used

[Yuri]

I discovered yesterday that '__cxa_deleted_virtual' function is missing 
in libcxxrt exports list, and therefore isn't available.
As a consequence, C++ ports using virtual function deletion will likely 
fail to build.
I had to add the copy of this function as a port patch to solve this, 
but I am not sure what the general solution might be when more ports 
will hit this problem.

Bug report: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=200863 
(Added missing '__cxa_deleted_virtual' into libcxxrt)

Yuri