Several tests are failing because of those missing symbols.

I understand why we need to export symbols relying in the versioned 
namespace but I don't understand why we need to do it for _GLIBCXX_DEBUG 
symbols which are not version namespace dependant.

Do you want to backport the Debug symbol ?

     libstdc++: Add mising gnu-versioned-namespace symbols

     libstdc++-v3/ChangeLog:

             * config/abi/pre/gnu-versioned-namespace.ver:
             Add __istream_extract and 
_Safe_local_iterator_base::_M_attach_single
             symbols.

Tested under Linux x86_64 versioned namespace.

Ok to commit ?

François