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

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis approved this pull request.

Hey Adam, thank you for all this work, LGTM! 

Any comments from Meta? (cc: @maksfb, @aaupov)

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