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) {

py/lexer.c

@@ -346,7 +346,8 @@ STATIC void parse_string_literal(mp_lexer_t *lex, bool is_raw) {
                                 vstr_add_char(&lex->vstr, '\\');
                                 break;
                             }
-                        // Otherwise fall through.
+                            // Otherwise fall through.
+                            MP_FALLTHROUGH
                         case 'x': {
                             mp_uint_t num = 0;
                             if (!get_hex(lex, (c == 'x' ? 2 : c == 'u' ? 4 : 8), &num)) {

py/mpconfig.h

@@ -1643,6 +1643,13 @@ typedef double mp_float_t;
 #endif
 #endif
 
+// Explicitly annotate switch case fall throughs
+#if defined(__GNUC__) && __GNUC__ >= 7
+#define MP_FALLTHROUGH __attribute__((fallthrough));
+#else
+#define MP_FALLTHROUGH
+#endif
+
 #ifndef MP_HTOBE16
 #if MP_ENDIANNESS_LITTLE
 #define MP_HTOBE16(x) ((uint16_t)((((x) & 0xff) << 8) | (((x) >> 8) & 0xff)))

py/objset.c

@@ -445,6 +445,7 @@ STATIC mp_obj_t set_unary_op(mp_unary_op_t op, mp_obj_t self_in) {
                 }
                 return MP_OBJ_NEW_SMALL_INT(hash);
             }
+            MP_FALLTHROUGH
         #endif
         default:
             return MP_OBJ_NULL;      // op not supported