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.

A few internal source files were redeclaring types and macros also
declared in the external jack/types.h and jack/systemdeps.h headers.
Since the external header needs them, delete them from the internal
headers.