Don't use nopl in cpus that don't support it.

  Patch by Mikulas Patocka. I added the test. I checked that for cpu names that
  gas knows about, it also doesn't generate nopl.

  The modified cpus:
  i686 - there are i686-class CPUs that don't have nopl: Via c3, Transmeta
	 Crusoe, Microsoft VirtualBox - see
	 https://bbs.archlinux.org/viewtopic.php?pid=775414
  k6, k6-2, k6-3, winchip-c6, winchip2 - these are 586-class CPUs
  via c3 c3-2 - see https://bugs.archlinux.org/task/19733 as a proof that
	 Via c3 and c3-Nehemiah don't have nopl