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

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis commented:

Hey Adam,

Thanks for addressing the comments!
`AArch64/perf2bolt-spe.test` seems to be failing. See comment below.
I've also added a few more nits and cleanups.



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