Left to its own devices, gcc calculates the full 64-bit product only to
discard the low 32 bits.  This forces it to do the right thing.

20% faster MP3 decoding on G4.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@18737 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b