[clang] [llvm] [RISC-V] Add support for MIPS P8700 CPU (PR #117865)

[Djordje Todorovic via cfe-commits]

djtodoro wrote:

> Extensions belong in their own PRs, ditto CPU/scheduler, driver and any additional passes. Each PR is one commit and this is not one commit's worth of changes.

@jrtc27 Yes! I agree. That is why I marked it as `Draft`. The next step is to divide it into multiple commits. Thanks.

https://github.com/llvm/llvm-project/pull/117865