gh-121040: Use __attribute__((fallthrough)) (#121044)

Fix warnings when using -Wimplicit-fallthrough compiler flag.

Annotate explicitly "fall through" switch cases with a new
_Py_FALLTHROUGH macro which uses __attribute__((fallthrough)) if
available. Replace "fall through" comments with _Py_FALLTHROUGH.

Add _Py__has_attribute() macro. No longer define __has_attribute()
macro if it's not defined. Move also _Py__has_builtin() at the top
of pyport.h.

Co-Authored-By: Nikita Sobolev <mail@sobolevn.me>

Python/formatter_unicode.c

@@ -320,7 +320,7 @@ parse_internal_render_format_spec(PyObject *obj,
                 format->thousands_separators = LT_UNDER_FOUR_LOCALE;
                 break;
             }
-            /* fall through */
+            _Py_FALLTHROUGH;
         default:
             invalid_thousands_separator_type(format->thousands_separators, format->type);
             return 0;