[Clang] __cpuidex probably should not be enabled for nvptx triples

https://github.com/llvm/llvm-project/pull/126324 introduced failures (discussed on the PR) in certain CUDA compilations. https://github.com/llvm/llvm-project/pull/152556 introduces a temporary patch, but we probably should not be enabling `__cpuidex` on NVPTX targets.