[clang] [llvm] [X86] Use generic CPU tuning when tune-cpu is empty (PR #83631)

[via cfe-commits]

AtariDreams wrote:

> Please check the CI - these failures look relevant
> 
> ```
> Failed Tests (4):
>   lld :: COFF/lto-cpu-string.ll
>   lld :: COFF/lto.ll
>   lld :: ELF/lto/cpu-string.ll
>   lld :: MachO/lto-cpu-string.ll
> ```

they are and I am getting to them.

https://github.com/llvm/llvm-project/pull/83631