After updating gentoolkit, I got numerous revdep-rebuild errors on 2 of my machines, please see https://bugs.gentoo.org/show_bug.cgi?id=504654#
After discussion with the gentoo devs it was obvious the problem was local. On those 2 machines there is no reference on the LDPATH to /usr/lib, and all of the libraries not found were located there. On the 3rd machine there is an 05multilib file in /etc/env.d which contains LDPATH=/usr/lib, sparing it from the same problems. It is the only reference on that machine.
I have solved the problem temporarily by adding LDPATH=/usr/lib in the /etc/env.d/99local file on both machines, and now run the much more critical gentoolkit-9999 (excellent, btw) with success.
I confess to ignorance regarding that linker stuff, but I have never tinkered in any way with it, so I am lost in understanding why these files are not included by default.