Add a hackish debugging facility to provide a bit of information about
reason for generated trap. The dump of basic signal information and 8
bytes of the faulting instruction are printed on the controlling
terminal of the process, if the machdep.uprintf_signal syscal is
enabled.

The print is the only practical way to debug traps from a.out
processes I am aware of. Because I have to reimplement it each time I
debug an issue with a.out support on amd64, commit the hack to main
tree.

MFC after: 1 week