libstdc++: Fix assigning nullptr to std::atomic<shared_ptr<T>> (LWG 3893)

libstdc++: Fix assigning nullptr to std::atomic<shared_ptr<T>> (LWG 3893)

LWG voted this to Tentatively Ready recently.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_atomic.h (atomic::operator=(nullptr_t)):
	Add overload, as per LWG 3893.
	* testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc:
	Check assignment from nullptr.

(cherry picked from commit a495b738e4a89a8104798d005fd09474bbb916ff)