In LinkageMapELF.py, there's a fallback mapping for multilib categories, which is used by the LinkMapElf class when NEEDED.ELF.2 does not contain multilib categories due to being generated by older portage. This mapping should be consistent with the mutilib categories generated by the compute_multilib_category function which was modified by commit b50bbcb70506254d66937dac376056bbf99b3fe9.
There's a patch in the following branch: https://github.com/zmedico/portage/tree/bug_550898 I've posted it for review here: https://archives.gentoo.org/gentoo-portage-dev/message/6ce361fa02f3869d1675356aa6ab5a85
@blueness: I'm adding you to CC since you've been involved with the introduction of multilib categories, and commit b50bbcb70506254d66937dac376056bbf99b3fe9 changes the ia_64 multilib category to ia64_64.
This is in the master branch: https://gitweb.gentoo.org/proj/portage.git/commit/?id=c3ef903d1c95ce28922a2d7dcb00cc7a1d690201
Released in portage-2.2.21