How do I do string comparison in BPF programs ?

I tried `__builtin_memcmp`, it compiles, but libbpf complains:

```
libbpf: failed to find BTF for extern 'memcmp': -2
Error: failed to open BPF object file: No such file or directory
```

Though I can use a bounded loop to bypass this, I want to know if there is a graceful solution. Thanks.

No repository file or test is named. Start with a minimal BPF program using __builtin_memcmp and reproduce the libbpf BTF error, then inspect how libbpf handles the memcmp extern and compare it with the bounded-loop workaround. Done should be a documented or implemented graceful way to perform string comparison in BPF programs.

Written by the indexing model from the issue text.