[llvm] [BOLT][AArch64] Introduce SPE mode in BasicAggregation (PR #120741)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

Just adding that we are in the process of upstreaming 'brstack' support for SPE, which would handle the `PBT` feature nicely for us, and would sit nicely within bolt sources. Once upstreamed, we can adapt the patch to work for the LBR-format.
(cc: @kaadam)

https://github.com/llvm/llvm-project/pull/120741