ARC: MWDT: workaround for libs with stackcheck

As of today we have libraries provided by MWDT build with
stackcheck enabled. So we have to provide dummy
_fstack, _estack to make it working.

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