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

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> This did not fix the FIXME. Checking Is64Bit is the correct fix. Checking HasX86_64 is useless, that's never set if there is no CPU provided. `HasX86_64` just means the CPU is a 64-bit capable CPU, not that 64-bit mode is enabled

Oh, right. The `generic` CPU will set `HasX86_64` too.

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