mdadm-3.3.1-r2 fails to build because currently the debian repositories point to: 3.3.2, not 3.3.1 http://ftp2.jp.debian.org/debian/pool/main/m/mdadm/ ERROR 404 NOT FOUND being the end result. Currently you can build mdadm-3.4.
It's still on the Gentoo mirrors of course.
fix your GENTOO_MIRRORS setting