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)

All users of POST_PACKED_STRUCTURE also include <jack/types.h>, so to
avoid code duplication, we move the definition into a single place.

Signed-off-by: Adrian Knoth <adi@drcomp.erfurt.thur.de>

This reverts commit 5fa50673fc.

Need to branch within submodule before making changes.