[clang] [llvm] [RISCV] Remove experimental from Zacas. (PR #83195)

[Wang Pengcheng via cfe-commits]

https://github.com/wangpc-pp approved this pull request.

LGTM.
Will it be in LLVM 18? Or we need more time to examine its robustness?

https://github.com/llvm/llvm-project/pull/83195