[clang] [llvm] [RISCV] Add processor definition for XiangShan-KunMingHu (PR #89359)

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Khao7342 wrote:

> Has KunMingHu's RTL been finalized (IIRC, we have vector unit under development)? And can we have different doc for different generations of XiangShan?

Thanks for your attention to Xiangshan. KunMingHu's RTL has not been finalized yet. The development work on vectors is in the final stages of testing and performance optimization. And there won't be too many changes on the RTL.

https://github.com/llvm/llvm-project/pull/89359