Zcu: allow atomic operations on packed structs

[jacobly0]

Same validation rules as the backing integer would have.

[SeanTUT]

This would probably be beneficial for std.atomic.Value, which currently wraps the underlying type in an extern struct, but this approach makes it impossible to use non ABI width integers/enums with a std.atomic.Value, whereas if a packed struct were used instead, non aligned bit widths could be used in atomics