[llvm] [RISCV] Add SiFive X390 scheduling model (PR #143938)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp commented:

Can you separate this PR into 2 PRs at least, it is massive and hard to review:
1. Adding abstraction.
2. Adding X380.

https://github.com/llvm/llvm-project/pull/143938