[clang] [llvm] [RISCV] Update P extension to 0.21 (PR #188121)

[Kito Cheng via cfe-commits]

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@@ -167,7 +149,7 @@ Changes to the RISC-V Backend
 
 * `llvm-objdump` now has support for `--symbolize-operands` with RISC-V.
 * `-mcpu=spacemit-x100` was added.
-* Change P extension version to match the 019 draft specification. Encoded in `-march` as `0p19`.
+* Change P extension version to match the 0.20 draft specification.
----------------
kito-cheng wrote:

```suggestion
* Change P extension version to match the 0.21 draft specification.
```

https://github.com/llvm/llvm-project/pull/188121