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

[Wang Pengcheng via cfe-commits]

wangpc-pp 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. :-)

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