[llvm] Add initial support for SPE brstack format (PR #129231)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

> Do you have plans to include an end-to-end (binary) test, with perf data with SPE samples?

Good idea! Testing on an fdata file directly maybe not cover everything. And running `perf2bolt` requires a recent perf version, which can be flaky across platforms.

Perhaps we could reuse/extend the `--perf-script-events` logic to accept a pre-generated text file instead.
One produced on a platform with a recent perf, eg:
```bash
perf script -F pid,brstack --itrace=bl ..
```

We could also add another in-tree unittest, that uses SPE with the PBT feature.
I could provide some sample output in the coming days/weeks.

Those additions can go in a follow-up PR, so we don't block this one?

https://github.com/llvm/llvm-project/pull/129231