ARC: MWDT: workaround missing ssize_t definition

ARC MWDT don't provide ssize_t type which is used in Zephyr.
Define ssize_t for ARC MWDT toolchain.

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