mpv: disable with ARC external toolchain

This package fails when building with ARC external toolchain

Marking this with special comment "# ARC toolchain issue" as the package
is to be enabled as soon as the issue with the ARC toolchain is
resolved.

Fixes:
  http://autobuild.buildroot.net/results/fb6/fb677a917545adee321bdcd2c2519c81326448c4//



Signed-off-by: Vlad Zakharov <vzakhar@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>