[clang] [llvm] [RISCV] Add processor definitions for XuanTie C910V2 and C920V2 (PR #174056)

[Pengcheng Wang via cfe-commits]

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

The datasheet on the website is `C920V3`, so is this PR linked to the right link?

https://github.com/llvm/llvm-project/pull/174056