New ARCv3 GNU toolchain uses hs6x mcpu value for hs6x CPUs, the older
one allows to use any mcpu value.
Update old 'arc64' mcpu to 'hs6x' to be aligned with new toolchain
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>