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

[Wang Pengcheng via cfe-commits]

wangpc-pp wrote:

> > LGTM in general, except one question: will zicbom and zicboz be in the final RTL?
> 
> You can find the full implementation of `zicbom` and `zicboz` here: [OpenXiangShan/XiangShan at ca18a0b](https://github.com/OpenXiangShan/XiangShan/commit/ca18a0b47b0e4089fd0dd1c085091cb90bf98f25). cc @poemonsense

Please update the official document then. :-)

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