Support BCC perf maps

[zuzzas]

1. New configuration parameter for counters: PerfMap. You can reference it via BPF_PERF_OUTPUT and perf_submit in the C code.
2. An oom kill example which utilizes perf maps to push OOM events.

[zuzzas]

Now that I think of it, there are other counting programs, such as cgroup.yaml, that can benefit from it. This approach removes the unbounded growth of BPF_HASHes.

[bobrik]

Intention looks good (if I understand it correctly), but some changes and docs are needed.

[zuzzas]

In addition to your latest comment, I think I'll add an OOM example with tracepoints. I believe it could be useful to illustrate both approaches to this problem in the examples. That's why I'll do it in a follow-up PR.

[bobrik]

Thanks!