PR libstdc++/57250 * config/abi/pre/gnu.ver: Export new symbols. * include/Makefile.am: Add new header. * include/Makefile.in: Regenerate. * include/bits/shared_ptr_atomic.h: Define atomic access functions. * include/std/memory: Include new header. * src/c++11/shared_ptr.cc (_Sp_locker): Define and use mutex pool. * testsuite/20_util/shared_ptr/atomic/1.cc: New. * testsuite/20_util/shared_ptr/atomic/2.cc: New. * testsuite/20_util/shared_ptr/atomic/3.cc: New. From-SVN: r217452
