BZ#13592: Fix x86-64 memset

[Roland McGrath]

> AFAIR practice for glibc so far has been that if a patch comes with a 
> changelog entry, it will go in with the patch. But if somebody else 
> needs to write the changelog entry and also needs to take care of 
> pushing the patch in, he will normally use his name as he takes 
> responsibility for it,

That is how it tends to go.  Of course, it's entirely reasonable instead to
tell the originator submitter how to write a ChangeLog entry and get one
from them so you can use it.