Fix bus error on ARM platforms

With the current code, packed structs lead to unaligned memory access,
thus causing SIGBUS on ARM.

Let's disable packing on ARM for now. Since it's only used for mixed
32/64bit jackd installations and we're not facing such a scenario on
ARM, there's little use to worry more atm.

(Commit message copied from jackd2's 2f7d512b, since it's the same
problem)