Function elisp-site-file-install cannot be used to install general elisp files; especially, it accepts only one filename argument. Please use elisp-install instead (this is well-documented in elisp-common.eclass, btw). Patch will follow.
Created attachment 168212 [details, diff] Proposed patch for idb-10.1.018.ebuild This will properly install the elisp files and an Emacs site-init file. I didn't bother to add code for byte-compilation of idb.el since the file is very short.
Created attachment 168214 [details] files/50idb-gentoo.el Site-init file for Emacs.
Fixed in cvs. Thanks.