llama.cpp Increases Minimum Buffer Size for SYCL USM Allocations to 4 GiB

The update raises the minimum buffer size from 1 GiB to 4 GiB for USM system allocations, based on real-world experiments with models like Qwen3.5-35B-A3B-Q8 exceeding available VRAM on devices like the B70. This change aims to prevent overcommitting device memory when running large model weights on systems with limited VRAM.