py: Fix compiling with debug enabled and make more use of DEBUG_printf.

DEBUG_printf and MICROPY_DEBUG_PRINTER is now used instead of normal
printf, and a fault is fixed in mp_obj_class_lookup with debugging enabled;
see issue #3999.  Debugging can now be enabled on all ports including when
nan-boxing is used.

py/gc.c

@@ -910,7 +910,8 @@ void gc_dump_alloc_table(void) {
     GC_EXIT();
 }
 
-#if DEBUG_PRINT
+#if 0
+// For testing the GC functions
 void gc_test(void) {
     mp_uint_t len = 500;
     mp_uint_t *heap = malloc(len);