Do I understand it correctly that Gentoo does not plan to push out a fix for this sooner than in 30 days?
glibc-2.22-r2 was released at the same time as glibc-2.21-r2 and it contains the fix for this issue. The Changelog merely says "misc upstream fixes" but I verified the relevant changes are there.
The GLSA is https://security.gentoo.org/glsa/201602-02