Implement a `perf script --dlfilter` backend

[Xyene]

Currently, we spend a lot of time sscan-ing textual output from perf script, and filter out branches within the same symbol in OCaml.

On recent (5.14+) perf installs, we could offload this to perf script --dlfilter, and have a short native object that does the filtering for us. Ref https://man7.org/linux/man-pages/man1/perf-dlfilter.1.html.

[cgaebel]

When we do this, we should make sure to maintain support for pre-5.14 perf. Older perfs are extremely common, and most people aren't willing to install a recent perf userspace to use with their older kernel.