A possible ebuild is available at http://data.gpo.zugaina.org/mva/dev-util/android-ndk/
commit 976f96c42f6d07fa7aaf51278faba1c47f2a87d8 Author: Jason Zaman <perfinion@g.o> Date: Sun Nov 20 17:38:44 2016 dev-util/android-ndk: bump to 13b Package-Manager: portage-2.3.0