py, extmod: Introduce and use MP_FALLTHROUGH macro.

Newer GCC versions are able to warn about switch cases that fall
through.  This is usually a sign of a forgotten break statement, but in
the few cases where a fall through is intended we annotate it with this
macro to avoid the warning.

py/gc.c

@@ -298,7 +298,8 @@ STATIC void gc_sweep(void) {
                 #if MICROPY_PY_GC_COLLECT_RETVAL
                 MP_STATE_MEM(gc_collected)++;
                 #endif
-            // fall through to free the head
+                // fall through to free the head
+                MP_FALLTHROUGH
 
             case AT_TAIL:
                 if (free_tail) {