[clang] [llvm] [RISCV] Update P extension to the 018 version of the spec. (PR #170399)

[Brandon Wu via llvm-commits]

4vtomat wrote:

I saw the new spec says: `Also, because extensions Zba and Zbb are now assumed to be required for Base P, this version removes these scalar instructions that exist in Zba and Zbb:
SH1ADD SEXT.B SEXT.H MIN MINU MAX MAXU REV8 CLZ CLZW`
does is mean `P` implies `Zba` and `Zbb`?

https://github.com/llvm/llvm-project/pull/170399