llext: disable for Harvard ARC variants

Some ARC processor configurations have separate memory for code (ICCM)
and for data (DCCM). Such configurations are unsuitable for LLEXT,
except for some quite special cases. For now, disable LLEXT and its
tests for these devices completely.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>