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

[Alex Bradbury via cfe-commits]

asb wrote:

> Xiangshan is of great famousness in China and there is already a community in which many individual developers and organiztions/companies like PLCT, T-Head have participated. So I think we needn't worry about the maintenance. :-)

Thanks for that extra context!

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