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

[Sam Elliott via llvm-commits]

lenary wrote:

@eaeltsin there are no reviews yet, but you could provide one :)

I haven't (yet) because I am not super familiar with the code affected any more, but the commit and the change in the test look good enough to land, from my perspective.

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