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

[Craig Topper via llvm-commits]

topperc wrote:

Even `Is64Bit` won't work because we haven't called `ParseSubtargetFeatures` yet, so no flags are set. The best we could do is inspect the arch from the Triple to detect 64-bit.

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