selftests/bpf: Emit top frequent code lines in veristat

Production BPF programs are increasing in number of instructions and states
to the point, where optimising verification process for them is necessary
to avoid running into instruction limit. Authors of those BPF programs
need to analyze verifier output, for example, collecting the most
frequent source code lines to understand which part of the program has
the biggest verification cost.

This patch introduces `--top-src-lines` flag in veristat.
`--top-src-lines=N` makes veristat output N the most popular sorce code
lines, parsed from verification log.

An example of output:
```
sudo ./veristat  --top-src-lines=2   bpf_flow.bpf.o
Processing 'bpf_flow.bpf.o'...
Top source lines (_dissect):
    4: (bpf_helpers.h:161)	asm volatile("r1 = %[ctx]\n\t"
    4: (bpf_flow.c:155)	if (iph && iph->ihl == 5 &&
...
```

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240930231522.58650-1-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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