[BUG]: libcu++ should support the three-way comparison operator

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this request and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Area

libcu++

### Is your feature request related to a problem? Please describe.

in `cuda/std/detail/libcxx/include/__config` there is this:

```c++
// FIXME: Correct this macro when either (A) a feature test macro for the
// spaceship operator is provided, or (B) a compiler provides a complete
// implementation.
# define _LIBCUDACXX_HAS_NO_SPACESHIP_OPERATOR
```

a lot of code is hidden behind the `_LIBCUDACXX_HAS_NO_SPACESHIP_OPERATOR` macro, which is set unconditionally. this needs attention.

### Describe the solution you'd like

properly support the three-way comparison operator when it is available.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_