The Gentoo ebuild for amavis 2.12.0 was committed two hours ago. It adds two new USE flags 'rspamd' and 'rspamd-https' which will take care of new Perl dependencies if you want to use the Rspamd extension.
As usual for updated ebuilds, it is considered unstable, so you need to use ">=mail-filter/amavisd-new-2.12.0 ~amd64" or similar. Please note that the package name remains mail-filter/amavisd-new for the time being. Renaming Gentoo packages causes extra work, and we're currently quite busy. -Ralph