[clang] [clang][RISCV][Zicfilp] Force user to use `-mcf-branch-label-scheme=unlabeled` (PR #152122)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

@mylai-mtk is no longer working on RISC-V, and we/SiFive will helping pushing CFI related PR forward after discuss with him offlist :)

And this PR I've checked on local, no rebase needed, so gonna merge this :)

https://github.com/llvm/llvm-project/pull/152122