[llvm] [Exegesis][RISCV] Add RISCV support for llvm-exegesis (PR #89047)

[Bushev Dmitry via llvm-commits]

dybv-sc wrote:

> If we're only supporting generating snippets in this patch rather than actually running them, the other pieces that are not necessary for snippet generation (like the RISCV counter class) should be dropped.
> 
> Dropping that also delays the discussion of post-commit testing infrastructure, which would be really ideal to have for this. The story isn't particularly great on X86 (one buildbot that breaks for long periods sometimes) due to the need for bare-metal access along with having libpfm installed. The requirements will be similar for RISCV, and it would be have a buildbot that will actually test snippet execution.

Agreed, removed performance counter for now.

https://github.com/llvm/llvm-project/pull/89047