mplayer: Add support for ARC

This patch adds support for ARC architecture in Mplayer's configure script.
Same patch has been submitted to upstream MPlayer. This patch should be
removed after Buildroot will bump to next version of MPlayer (after 1.1.1).

Fixes http://autobuild.buildroot.net/results/f62/f62ace7ba49d492ea224bb8f3beb547389aab517/



[Peter: add autobuilder link]
Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>