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

[brycelelbach]

Today, we #if away atomic_flag's wait/notify API for < SM70, 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.