yasm does support x32 ABI right now.
Created attachment 324606 [details, diff] yasm-9999.ebuild.diff
the python logic probably needs fixing
should be all set now in the tree; thanks for the report! Commit message: Add live git ebuild http://sources.gentoo.org/dev-lang/yasm/yasm-9999.ebuild?rev=1.1
Commit message: Add x32 support from upstream http://sources.gentoo.org/dev-lang/yasm/yasm-1.2.0-r1.ebuild?rev=1.1