[clang] [RISCV] Alphabetize riscv_files in clang/lib/Headers/CMakeLists.txt. NFC (PR #184024)

[Pengcheng Wang via cfe-commits]

https://github.com/wangpc-pp commented:

This is why I said the naming is confused...

https://github.com/llvm/llvm-project/pull/184024