[X86] Infer 64bit feature support from the CPUID results in getHostCPUFeatures.

After r341022, we more strictly check the 64bit feature in X86Subtargets constructor when a 64-bit triple is used. If we don't infer this feature for autodetected CPUs we might incorrectly report an error if the CPU name wasn't autodetected to a CPU that supports 64-bit.

llvm-svn: 342914