FIX: [droid;x86;ffmpeg] disabling mmx on recent gcc actually causes crashes

I don't know what was the original issue, but not enabling mmx (on gcc 4.8) surely causes crashes when playing, e.g., wmv, avi, ...

Reverts https://github.com/xbmc/xbmc/commit/be07e097e5be70cec0293bd56e04231011938d6c
Adds a build time dependency on yasm.

/cc @davilla