Add 3dnow intrinsic support for AMD64 machines: -march=athlon means IA32 for GCC, so use -m3dnow instead.
Note that there's no way to specificly activate 3dnow ext support


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@5380 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b