sys-apps/systemd-9999 requires >=sys-apps/util-linux-2.26, otherwise activating swap .mount units won't work anymore. See also: http://cgit.freedesktop.org/systemd/systemd/commit/README?id=bf1d7ba70aceddb5dae0cd2e370b8afaf0c81b05
Fixed in https://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/sys-apps/systemd/systemd-9999.ebuild?revision=1.170&view=markup