[clang] [llvm] [RISCV][MC] Implement MC for Base P extension (PR #123271)

[via llvm-commits]

realqhc wrote:

> Can we break this up into smaller patches? The P spec is rather large. Reviewing 1000 lines of RISCVInstrInfoP.td in a single PR is very time consuming and exhausting as a reviewer.
> 
> The changes to RISCVFeatures.td and RISCVInstrInfoZb.td and other things that go with that could be the first patch. That should be easy to review and get landed.

I've created a separate patch https://github.com/llvm/llvm-project/pull/127160 with the changes. 

https://github.com/llvm/llvm-project/pull/123271