ARC: MWDT: fix include path for the case of C/C++ lib usage

Fix include path of headers provided by MWDT toolchain for the
case of MWDT C/C++ lib usage.

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