ARC: save/restore accumulator registers on all ARCv2 HS CPUs by default

Accumulator registers (ACCL, ACCH) are used on HS CPUs not only
in case of FPU usage but also in case of MPY usage. We enable MPY
for all ARCv2 HS in commit
18a24c3f ARC: gcc-m-cpu: use -mcpu=archs as a default for ARCv2 HS
but we didn't enable accumulator registers management.

Let's enable accumulator registers save/restore on all ARCv2 HS CPUs
by default.

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