[llvm] [RISCV] Allow spilling to unused Zcmp Stack (PR #125959)

[Sam Elliott via llvm-commits]

lenary wrote:

I'm back at my desk on Thursday/Friday, we can revert temporarily and I can pick this up again then if you can get a testcase?

It sounds like to access the RVV area, we're adding the wrong amount to `SP`, and maybe padding it incorrectly, but I'm not sure.

https://github.com/llvm/llvm-project/pull/125959