[clang] [llvm] [RISCV] Add Qualcomm uC Xqcia (Arithmetic) extension (PR #118113)

[Sam Elliott via cfe-commits]

https://github.com/lenary approved this pull request.

LGTM.

I note Craig has questions about the spec, we will ensure those are relayed to the spec authors, but they don't relate to encoding/assembling problems, so I'm not sure if they should block landing this.

https://github.com/llvm/llvm-project/pull/118113