[llvm] [PREVIEW-ONLY] RVV support for llvm-exegesis (PR #114149)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

>  I'm creating raw Perf events by myself rather than using libpfm. I thought it's pretty east to do so even with the existing code in llvm-exegesis, so I'll send another PR to add this support

This seems reasonable enough to me given that libpfm doesn't seem to support the platforms that you're working on. Bringing up new platforms will also require using raw event encodings as libpfm will definitely not have support for those, so reasonable enough to me even if I would prefer to avoid it.

>  The -start-before-phase and -stop-after-phase features, as well as the object file serialization supports

Do we need a new `-stop-after-phase` flag? It kind of seems equivalent to the existing `--benchmark-phase` flag.

https://github.com/llvm/llvm-project/pull/114149