It seems as though recent versions of libelf are now the elfutils package instead. The recent addition of fwupd seems to depend explicitly on libelf, rather than elfutils, causing several blockers. It might be worth converting to elfutils (assuming it's just a dependency change rather than a code change, I don't know meson very well unfortunately)? If dev-libs/libelf is needed explicitly, then the dependency could be virtual/libelf-0 which should ensure the real libelf package is used instead of elfutils.
https://github.com/gentoo/gentoo/pull/5067
Sorry, I linked to the wrong PR. The correct one is: https://github.com/gentoo/gentoo/pull/5068