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

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> You need to add some `REQUIRES` to tests. @dybv-sc

Don't worry, I reverted this PR in bf62ea40eee82794abc8ed767c150d6c8d0c0b0a.
Please add some `REQUIRES`s and reland this. :-)

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