powerpc: stash off srr0 in si_addr for signals

si_addr is the address of the instruction executing at the time the
signal was sent.  Populate this field with srr0, which, though not
always the case, is most often the instruction that triggered the fault.

sys/powerpc/powerpc/trap.c

@@ -438,7 +438,7 @@ trap(struct trapframe *frame)
 		ksiginfo_init_trap(&ksi);
 		ksi.ksi_signo = sig;
 		ksi.ksi_code = (int) ucode; /* XXX, not POSIX */
-		/* ksi.ksi_addr = ? */
+		ksi.ksi_addr = (void *)frame->srr0;
 		ksi.ksi_trapno = type;
 		trapsignal(td, &ksi);
 	}