x86: fold CpuLM and Cpu64

x86: fold CpuLM and Cpu64

Now that CpuLM is used solely in cpu_arch_flags and cpu_arch[] while
Cpu64 is solely used in insn templates, they no longer need to be
treated different from other "ordinary" flags; the only "unusual" one
left if CpuNo64. Fold both, leaving just Cpu64.