audio/jack: Fix alignas() on non-packed architectures

Unbreak the build with Clang on architectures where JACK uses non-packed
data structures, like arm64 or powerpc. The alignment errors are exposed
there because:

• The non-packed data structures require 8 byte alignment.
• alignas() is not allowed to weaken alignment requirements to 4 bytes.
• Clang enforces this according to standard, GCC ignores it.

Use an additional alignas() specifier to let the compiler choose between
minimum alignment (packed) and "natural" alignment (non-packed). This is
both standard c++11 and compatible with older GCC, which does not
propagate alignment requirements of packed substructures properly.

PR: 261508
Submitted by: Florian Walpen <dev@submerge.ch>