Add a way to see all possible values for `-fsycl-targets` flag

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

This is split from #1191 where folks were confused about which targets are supported by our SYCL implementation and which spelling should be used for them, i.e. what are the possible values for `-fsycl-target` compiler flag?

### Describe the solution you would like

There is no proposal for this yet and the solution/interface should be developed as part of this issue. Original issue mentioned `-fsycl-targets=list` as an example of how it could be used. Displaying the full list under `--help` is also an option

### Describe alternatives you have considered

_No response_

### Additional context

_No response_