fix traceback prints - %S was not advancing pointer enough

R=r
DELTA=6  (5 added, 1 deleted, 0 changed)
OCL=27500
CL=27525

src/runtime/print.c

@@ -58,11 +58,15 @@ printf(int8 *s, ...)
 			break;
 		case 'p':	// pointer-sized
 		case 's':
-		case 'S':
 			if(sizeof(uintptr) == 8 && ((uint32)(uint64)arg)&4)
 				arg += 4;
 			narg = arg + sizeof(uintptr);
 			break;
+		case 'S':	// pointer-aligned but bigger
+			if(sizeof(uintptr) == 8 && ((uint32)(uint64)arg)&4)
+				arg += 4;
+			narg = arg + sizeof(String);
+			break;
 		}
 		switch(*p) {
 		case 'd':