MFC r314061:

Add __int128-related symbols to libcxxrt's version map. Put these into
the same CXXABI verions as recent libstdc++.

Note that __int128 types are only available on arches where long long is
128 bit wide.

Noticed by: harti

MFC r314104:

Surround any unmangled C++ names in libcxxrt's version map with 'extern
"C++"', otherwise ld refuses to make the symbols global in the final
library. This causes the __int128-related symbols to go missing when
the library is stripped during installation.

Helpful hints: emaste