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

[Simon Pilgrim via cfe-commits]

RKSimon 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
```

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