A new release of nasm came out around 3 weeks ago: http://www.nasm.us/pub/nasm/releasebuilds/2.13.01/ It is required for building media-libs/x264-9999
I tried copying/renaming the existing nasm-2.12.01.ebuild as nasm-2.13.01.ebuild in my local overlay and it failed on the 'emake nasmlib.o' line. That target is apparently no longer in the Makefile. I removed that line and nasm-2.13.01 build successfully. x264-9999 now builds using the new nasm.
Confirmed here as well. Bump with the line delete works for me.
slightly offtopic, but why does media-libs/x264-9999 keyworded? You are stepping into this kind of rised dependency requirements in live ebuilds quite easily. For an unstable keyworded ebuild it make sense to provide a snapshot versioned ebuild based on SHA commits or any relevant way.
commit b7a831e509fcfc0300fc6d026de2c65934667fe3 Author: Louis Sautier <sautier.louis@gmail.com> Date: Thu Jun 15 21:14:42 2017 +0200 dev-lang/nasm: bump to 2.13.01, EAPI=6, fix doc dependencies Bug: https://bugs.gentoo.org/show_bug.cgi?id=619360