virt68k: trap: Fix ksi_code for T_TRAP{,15}|T_USER

Fix ATF failures for lib/libc/sys/t_ptrace_wait*:{,set}step*.

XXX
It would be really nice if we can factor out copy-paste b/w
m68k ports...

branches: 1.3.2;
Don't crash in copyin/copyout when a NULL pointer is passed.

Largely unify the <machine/cpu.h> headers on the m68k platforms.

Add a port to the Qemu m68k "virt" platform, a 68040-based virtual machine
with VirtIO I/O devices.

This is a work-in-progress, but "successfully mounts root file system
and then complains about missing /sbin/init that is, in fact, not there"
seems like a good checkpoint.