refactor: simplify xllm_ops pre-build checks and marker-based rebuild logic.

[LMX-xin]

Summary

• skip xllm_ops precompilation when the installed marker matches the source git HEAD
• trigger xllm_ops precompilation only when the marker is missing or stale
• add a CANN version guard before rebuilding xllm_ops, and require CANN 8.5 for compilation

[gemini-code-assist]

Code Review

This pull request refactors the pre-compilation logic for xllm_ops in CMakeLists.txt and utils.py. The changes introduce a new mechanism using a .xllm_ops_git_head marker file to track the third_party/xllm_ops git HEAD, replacing the previous CMake cache variable approach. This ensures xllm_ops is rebuilt only when its source code changes or the marker is missing. Additionally, the Python script now includes robust error handling for git operations and enforces a CANN version 8.5 requirement for compilation. The review comments point out a potential inconsistency in both CMakeLists.txt and utils.py regarding the precedence of ASCEND_HOME_PATH and ASCEND_OPP_PATH when determining the opp root, suggesting that ASCEND_OPP_PATH might be expected to have higher priority.