[llvm] [Exegesis][RISCV] Support C_LDSP for llvm-exegesis (PR #169660)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> I guess RISC-V is actually RISC so you do not get instructions like `pushq` or `popq`

tangent to this PR: the Zcmp extension actually has stack push / pop, they're called `cm.push` and `cm.pop`

https://github.com/llvm/llvm-project/pull/169660