Fix the second-stage max reduction in the SYCL softmax kernel when a workgroup contains more subgroups than WARP_SIZE.

Previously, after each subgroup wrote its partial max to buf_iw[warp_id], the cross-subgroup reduction only loaded:

max_val = buf_iw[lane_id];

This only reduces the first WARP_SIZE partial maxima. For example, on Intel GPUs, WARP_SIZE can be 16 while the softmax workgroup size is 1024, producing nwarps = 64. In that case, partial maxima from buf_iw[16..63] were ignored.
If the true row max is in one of the ignored subgroups, softmax subtracts a too-small max value. For large attention logits this can overflow exp(x - max) to inf, eventually producing NaN.

llama.cpp PR: ggml-org/llama.cpp#24451