[FEA]: memcpy_async global->global on system-scope barrier

### 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.

sm_90+ supports global->global asynchronous copies on system-scope barriers.

### Describe the solution you'd like

See above.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_