[llvm] [RISCV] Add mvendorid/marchid/mimpid values for SiFive P870-D. (PR #216106)

[Pengcheng Wang via llvm-commits]

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

We can add a test in Clang (I can't remember the name but you may search existing CPUs with these IDs).

https://github.com/llvm/llvm-project/pull/216106