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

[Yinan Xu via cfe-commits]

poemonsense wrote:

> Please update the official document then. :-)

I'll do it. We did not list it previously because it is not the usual thing for user programs, as we were initially implementing it for internal usage. Sorry for causing the confusion.

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