update killsnoop to use tracepoints

This is a request for help.

I wrote killsnoop back in 2015 before tracepoint support, and so I kprobe'd sys_kill(). It still does some derivation of that. But now there's a report it no longer works on Linux 5.11: https://github.com/iovisor/bcc/pull/3572#issuecomment-900357032 CC @chenhengqi

Can someone please update killsnoop (both Python and libbpf-tools) to use tracepoints instead of kprobes (if it works as expected). All of these:

  syscalls:sys_enter_kill                            [Tracepoint event]
  syscalls:sys_enter_tgkill                          [Tracepoint event]
  syscalls:sys_enter_tkill                           [Tracepoint event]
  syscalls:sys_exit_kill                             [Tracepoint event]
  syscalls:sys_exit_tgkill                           [Tracepoint event]
  syscalls:sys_exit_tkill                            [Tracepoint event]