=x11-misc/lightdm-gtk-greeter-2.0.1-r1 compiles and runs fine on PowerPC 64 architecture. Tested with an Imac G5 machine. I suggest to add ~ppc64 keyword following ppc one, as the two are the same architecture.
~ppc64 keywords was added, closing. commit 1ddbdf8dd398085a159b58223e5e2d1c8916d082 Author: Jeroen Roovers <jer@gentoo.org> Date: Sat Oct 15 07:34:11 2016 +0200 x11-misc/lightdm-gtk-greeter: Mark ~ppc64 (bug #588202). Package-Manager: portage-2.3.2 RepoMan-Options: --ignore-arches