sysinfo: check if cpu has split lock detection flag

[jrelvas-ipc]

Check if the cpu has split lock detection, then output accordingly with the sysinfo command. This helps users diagnose if their machine is affected by the split lock trap issue.

Go's cpu package does not expose this "flag" (seems to not actually be a real cpu flag, just something made up by the linux kernel), so it must be read directly from cpuinfo.

Documentation will be updated accordingly.

Note: Unsure if "Supports split lock detection" or "Has split lock detection" is better. Let me know.