sycl: add GGML_SYCL_USE_ASYNC_MEM_OP env toggle

[aicss-genai]

Overview

Authors

• @unrahul
• @ctao456
• @malsbat
• @YuningQiu
• @vvravi1
• @zhiqitao-intel

Exposes async USM allocation/free as an independent env knob.

Previously g_ggml_sycl_use_async_mem_op was implicitly tied to whether graphs were enabled (= !g_ggml_sycl_disable_graph). The reorder staging path benefits from async alloc/free even without graphs — it avoids host waits while preserving queue ordering.

Adds GGML_SYCL_USE_ASYNC_MEM_OP (default 1) read at init, logged alongside the other SYCL env vars. The SYCL extension availability check is unchanged.

Additional information

Split from #22066 per reviewer request for independent review.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: Yes. This work was partially produced with an agentic engineering approach: agents surface issues and explore experiments while engineers identify and reject candidates using domain knowledge. Human feedback involved.

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