[llvm] [RISCV] fix SP recovery in varargs functions (PR #114316)

[Kito Cheng via llvm-commits]

kito-cheng wrote:

It's LGTM and I will give formal LGTM once https://github.com/llvm/llvm-project/pull/114970 land and this PR rebased

https://github.com/llvm/llvm-project/pull/114316