[clang] [RISCV] Add processor definition for XiangShan-NanHu (PR #70294)

[Yinan Xu via cfe-commits]

poemonsense wrote:

> LGTM in general, except one question: will zicbom and zicboz be in the final RTL?

Yes, it's in the final RTL.

https://github.com/llvm/llvm-project/pull/70294