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

[via llvm-commits]

AnastasiyaChernikova wrote:

@dtcxzyw Yes, I work with real riscv boards. This code is tested on licheepi and thead.

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