I've missed it during stabilization. Could you please backport: http://git.videolan.org/?p=ffmpeg.git;a=commit;h=d07950f774e03503146b299817941566453309bd to current stable?
+ 23 Oct 2011; Kacper Kowalik <xarthisius@gentoo.org> + +files/ffmpeg-0.7.6-fix_ppc64_32ul.patch, ffmpeg-0.7.6.ebuild: + (non-maintainer commit) Backport patch that allows build system to properly + detect arch on ppc64 with 32ul. Fixes bug 341235 and 387207