libstdc++: Add pretty printer for std::atomic

libstdc++: Add pretty printer for std::atomic

For the atomic specializations for shared_ptr and weak_ptr we can reuse
the existing SharedPointerPrinter, with a small tweak.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (SharedPointerPrinter): Add
	support for atomic<shared_ptr<T>> and atomic<weak_ptr<T>>.
	(StdAtomicPrinter): New printer.
	(build_libstdcxx_dictionary): Register new printer.
	* testsuite/libstdc++-prettyprinters/cxx11.cc: Test std::atomic.
	* testsuite/libstdc++-prettyprinters/cxx20.cc: Test atomic smart
	pointers.