sycl: Increase minimum buffer size for USM system allocations (#25525)

Raise the threshold for minimum buffer size from 1 GiB to 4 GiB, based on real-world experiments of overcommitting device memory with model weights larger than available VRAM, for example Qwen3.5-35B-A3B-Q8 running on a B70.

Also add a debug message to better track USM system allocations.

Signed-off-by: Francois Dugast [email protected]