Use /etc/portage/package.keywords (with an actual keyword, instead of the
default ~ARCH) or your local overlay.
However, a missing keyword (like neither amd64 nor ~amd64) indicates that
the ebuild has not been tested on that architecture. If the ebuild DOES
work, report it on b.g.o. (Actually, even if it doesn't, you should
probably report it, because then the -ARCH keyword can be added which
means "tested and does not work".)