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

[via llvm-commits]

AnastasiyaChernikova wrote:

The processInstructionReservedRegs function has been removed, as rewriting it to a more general form is still difficult. We will add support for it in future pr. Its absence does not allow us to support only a few instructions, which I don't think is critical for the first patch. At the moment, I have responded to all comments. If there are no more comments, could merge this pr?
@boomanaiden154 @gchatelet @mshockwave @topperc @michaelmaitland @legrosbuffle @wangpc-pp @dtcxzyw @keeranroth

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