Don't provide `atomic` wait/notify for < SM70

Today, we [`#if` away `atomic_flag`'s wait/notify API for < SM70](https://github.com/NVIDIA/libcudacxx/blob/feature/nvcxx-compatibility/libcxx/include/atomic#L2600), but we don't do the same for `atomic`. We should be consistent here.

We should also consider whether `#if` away is the right solution. The C++ committee decided to prefer declaring, but not defining methods that can't be supported in freestanding; this makes metaprogramming/SFINAE a little more sane and consistent. We should consider this option.