[patch] libstdc++/57250 shared_ptr atomic operations

[Jonathan Wakely]

I'm not very proud of this solution, but unless anyone has a better
ideas this is how I plan to add the atomic operations for shared_ptr.

I used __gnu_cxx::__mutex instead of std::mutex because it has fewer
dependencies, so the atomic operations should always be available for
shared_ptr even when the target doesn't support C++11 threads.

The operations are also defined for the __shared_ptr class template.

Tested x86_64-linux.
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