[clang] [RISCV] Add sched model for XiangShan-NanHu (PR #70232)

[Philip Reames via cfe-commits]

preames wrote:

Can you separate out the basic processor definition (using NoSchedModel), and a patch which adds the scheduling model?  We can at least get the processor definition landed while we iterate on the scheduling related pieces.  

https://github.com/llvm/llvm-project/pull/70232