ffmpeg: disable mmx for low end x86-type processors

ffmpeg enables mmx by default for x86-class processors, so disable it for
processors that lack it (i386, i486, i586, i686, pentium pro, geode).
Solves bug #3325

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>