[ia32] cpu capabilities cleanups and additions

[ia32] cpu capabilities cleanups and additions

* Add support for new Centaur(VIA) and Intel cpuid feature bits,
  expanding the x86_capability array by two.
* (cleanup) Move cpu setup for newer Via C3 cpus into its own
  function, init_c3()
* Add support for RNG control msr on VIA Nehemiah
* export X86_FEATURE_XSTORE and cpu_has_xstore macros so that
  kernel code may easily test for cpu support of the new
  "xstore" instruction.