If the user leaves arch set as generic and doesn't set -custom-cflags our kernels fail now. IDK if you want to do some checking in the ebuild or some documentation on the wiki for a fix. Some users leave arch always as generic and setup flags in make.conf when they have one off procs that are missing an instruction, normally like AES. These users will never be able to take advantage of this optimization as it right now. Others for some reason just leave everything generic which just seems weird to me. I noticed this problem from a current forum post. Thanks for looking.