Better discoverability of allowlist support

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

I'm getting back into SYCL after a few years, and testing the Intel OneAPI implementation (from official .deb, version 2025.0) I was surprised to find out that `sycl-ls` (and my own custom `syclinfo`) would only show the Intel OpenCL platform, despite me having two other platforms (rusticl and pocl) that properly declare SPIR-V support. I also tried running with `SYCL_UR_TRACE` at various level, and I could see that the relevant platforms were being probed, but without any information about why they were being discarded.

Ultimately @bashbaug pointed me at the documentation for `SYCL_DEVICE_ALLOWLIST` ([relevant Fediverse thread][fedithread]) and I was able to have them show up by setting it to the empty string.

[fedithread]: https://fediscience.org/@giuseppebilotta/114024704988196579/

### Describe the solution you would like

The presence of an allowlist (particularly a built-in one) should be better advertised. At the very least, a message on the reason why a platform/device is being discarded should be given in the trace.

(Please let me know if this is the wrong component to report this issue against, so that I may move it to a more appropriate one.)

### Describe alternatives you have considered

_No response_

### Additional context

_No response_