Implement randUint64() directly in the maphash overlay to ensure compatibility across Go versions. Go 1.21's randUint64() calls runtime.fastrand64, while Go 1.22+ calls runtime.rand. The wrapper function bridges to llgo's fastrand64 implementation, avoiding undefined symbol errors on Go 1.21. Generated with [codeagent](https://github.com/qbox/codeagent) Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>