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

[Phoebe Wang via llvm-commits]

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

> @phoebewang What do you think?

This happens only when both `CPU` and `TuneCPU` are empty. Front ends like Clang always sets a valid `CPU`, while most llc tests are not sensitive to tuning targets (the rest already set target-cpu/tune-cpu). So I think the change doesn't have much value in reality.

Nevertheless, I think it's good to reduce one FIXME, so LGTM.

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