mplayer: not supported on ARC

Fixes:

  http://autobuild.buildroot.org/results/af1/af1ca869dbc7a71aa4cc89373d605ca8f790fcc9/



[Peter: also adjust largefile comment dependency]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>