toolchain: arcmwdt: always enable TLS in case of CPP

The CPP in ARC MWDT toolchain relies on TLS (THREAD_LOCAL_STORAGE)
so enable it if CPP is selected.

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