tests: ARC: enable back tests/lib/ringbuffer on qemu_arc_hs6x

tests/lib/ringbuffer is now finally fixed with commit 15e834a6
("linker: __data_region_start equal to __data_start"), so we can enable
it back.

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