x86, cpufeature: Catch duplicate CPU feature strings

We had a case of duplicate CPU feature strings, a user space ABI
violation, for almost two years.  Make it a build error so that
doesn't happen again.

Link: http://lkml.kernel.org/r/4FE34BCB.5050305@linux.intel.com
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Jean Delvare <khali@linux-fr.org>