Only minor changes are required to make the kernel 64 bit compatible.
Most of the changes are either DEBUG/printf formatting or different types for void pointer casting.
The only other change is the type of the `data` member in priority_queue_node_t, as `data` must be able to store a pointer.
For current architectures, the assumption `sizeof(unsigned int) == sizeof(void *)` holds, but not for 64 bit.
Therefore, the type is changed to `uintptr_t', which has the same size for the current architectures, but can also store a pointer in 64 bits.
The mbox code contains a race condition in `mbox_put()`: When it
waits for a slot in the queue to become available, it is woken up with
IRQs enabled. It disables IRQs again as first thing, but by then
another thread may already have preempted the running thread and filled
the queue back up. In this case, a message in the queue would be
silently overwritten.
Replace accesses to `sched_active_thread`, `sched_active_pid`, and
`sched_threads` with `thread_get_active()`, `thread_get_active_pid()`, and
`thread_get_unchecked()` where sensible.
