ARC: MWDT: LIB: implement _istty hook

Implement _istty hook as it is required for proper setup of
STDIN/STDOUT/STDERR buffering.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>