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

[Craig Topper via llvm-commits]

topperc wrote:

This did not fix the FIXME. Checking Is64Bit was the correct fix. Checking HasX86_64 is useless, that's never set if there is no CPU provided.

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