std::shared_ptr atomic operations

	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