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

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM

Note that I'm not reviewing the correctness of the schedule model at all, I'm purely glancing at code structure and testing to make sure this doesn't impact the backend in general.  

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