teslibct: don't allow picolib tests for ARC MWDT toolchain

Currently picolib isn't compatible with ARC MWDT toolchain,
so don't try to build picolib tests in case of ARC MWDT toolchain
usage.

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