[clang] [llvm] [RISCV] Mark Xqccmp as not experimental (PR #175066)

[Sudharsan Veeravalli via llvm-commits]

https://github.com/svs-quic commented:

Could you please add a release note for this? 

Also could you update the extension name in RISCVUsage.rst as well?
```
``experimental-Xqccmp``
  LLVM implements `version 0.3 of the 16-bit Push/Pop instructions and double-moves extension specification <https://github.com/quic/riscv-unified-db/releases/tag/Xqccmp_extension-0.3.0>`__ by Qualcomm. All instructions are prefixed with `qc.` as described in the specification.
```

https://github.com/llvm/llvm-project/pull/175066