Add FALLTHROUGH comments as needed

I investigated these cases and confirmed that the fallthrough behavior
was intentional.

extmod/re1.5/recursiveloop.c

@@ -22,6 +22,7 @@ recursiveloop(char *pc, const char *sp, Subject *input, const char **subp, int n
 		case Char:
 			if(*sp != *pc++)
 				return 0;
+                        /* FALLTHROUGH */
 		case Any:
 			sp++;
 			continue;

py/objset.c

@@ -450,6 +450,7 @@ STATIC mp_obj_t set_unary_op(mp_unary_op_t op, mp_obj_t self_in) {
                 return MP_OBJ_NEW_SMALL_INT(hash);
             }
 #endif
+            /* FALLTHROUGH */
         default: return MP_OBJ_NULL; // op not supported
     }
 }