Fix: Align buffers from objstack_alloc on sizeof(void *)

The buffers from objstack_alloc will store pointers, so they must
be aligned on a pointer's size, or else it will cause issues on the
CPUs which do not support unaligned addresses access.

Signed-off-by: Fredrik Markstrom <fredrik.markstrom@gmail.com>
Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>