[clang] allow prefer 256 bit attribute target (PR #117092)

[Phoebe Wang via cfe-commits]

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

LGTM.

Note, `prefer-256-bit` just prefers. If you want to 256-bit guaranteed, you need `no-evex512`.

https://github.com/llvm/llvm-project/pull/117092