libtorrent: disable on ARC arch

Libtorrent has dependencies on atomic operations. However ARC GCC doesn't
implement those at the moment so package should be disabled. Rtorrent
depends on libtorrent.

This fixes:
http://autobuild.buildroot.net/results/51414e22c8ea47c38df20d864526a370c5a895d7/



Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>