ARC: arch.h: setup ARCH_STACK_PTR_ALIGN correctly for 64 bit

Stack is aligned by 8 for 64 bit ISA and 4 for 32 bit ISA.

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