Fixes https://github.com/jackaudio/jack1/issues/84

Change-Id: I5489729c1540647421258c66c2e74f321d50b777

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.