ARC: MWDT: link mwdt libc only if it is required

We don't need MWDT libc if we are using minimal one provided by
Zephyr. So let's avoid linking mwdt libc if CONFIG_MINIMAL_LIBC
is enabled.

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