[PATCH] Use cxx11 abi in versioned namespace

[François Dumont]

Hi

Here is the patch to use cxx11 abi for the versioned namespace mode.

It is still suffering from a side effect on gdb pretty printers with 3 
errors like:

got: type = 
std::unique_ptr<std::vector<std::unique_ptr<std::list<std::__8::basic_string<char, 
std::__8::char_traits<char>, std::__8::allocator<char> >>[]>>[99]>
FAIL: libstdc++-prettyprinters/80276.cc whatis p4

However once I apply the version bump errors are gone.

Let me know if it is ok to apply all at once or if you prefer to avoid 
the version bump.

François
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