Optimize clang-format check in Jenkins CI (#3597)

This change improves the clang-format CI check to be faster and not
depend on git being available in the build environment.

Changes:
- Use `find` instead of `git ls-files` (no git dependency)
- Check all C++ files: *.h, *.hpp, *.cpp, *.h.in, *.hpp.in, *.cpp.in, *.cl
- Exclude build/ and include/rapidjson directories
- Use parallel processing with 8 cores (-P 8) for ~8x speedup
- Show only errors with unified diff format (-u)
- Clear error messages: "ERROR: <file> needs formatting"
- Preserve original logic: run clang-format only when RUN_CPPCHECK=false,
  or run both clang-format and cppcheck when RUN_CPPCHECK=true

Performance:
- Sequential processing: ~93 seconds for 5,899 files
- Parallel with 8 cores: ~12 seconds for 5,899 files
- Per-file processing time: ~15ms

This reduces CI time while maintaining code formatting standards.