debian stretch: upgrade 32-bit to 64-bit

There are various reasons why one would prefer 64-bit over 32-bit (or not); basically it is about improved ABI (passing arguments in registers), bigger register (can be faster) versus higher memory usage (because pointers are twice as big). In some corner cases you want 64-bit to be able to use more memory in your programs (32-bit kernels can often handle more than 4G memory, but 32-bit userspace…