ARC: ARcv3: 64bit: manage accumulator reg properly

In case of ARCv3 64 bit we have only one 64bit accumulator
register instead of register pair, so fixup register
save & restore code.

While we at it also make ARC_HAS_ACCL_REGS option (which
controls accumulator reg/regs save & restore) default
for HS5x and HS6x as well - as it should be.

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