libbpf-tools: add CO-RE 'syncsnoop'

[myhro]

This is a CO-RE port of the Brendan Gregg's syncsnoop BCC Python tool. The code is heavily inspired by the runqslower port, with variables naming and comments style borrowed from other CO-RE tools.

I tried to make it look like existing tools as much as possible, but if there's anything missing please let me know.