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

[via llvm-commits]

AnastasiyaChernikova wrote:

@boomanaiden154 @gchatelet @mshockwave @topperc @michaelmaitland @legrosbuffle @wangpc-pp @dtcxzyw @keeranroth
I answered all the comments, please take a look. Maybe I could merge all the commits into one? 

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