[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?
> 
> This comment has not been addressed [#89047 (comment)](https://github.com/llvm/llvm-project/pull/89047#issuecomment-2484180781)

@topperc Added descriptions of the changes

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