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/bytecodes.c

@@ -885,10 +885,10 @@ dummy_func(
             switch (oparg) {
             case 2:
                 cause = PyStackRef_AsPyObjectSteal(args[1]);
-                /* fall through */
+                _Py_FALLTHROUGH;
             case 1:
                 exc = PyStackRef_AsPyObjectSteal(args[0]);
-                /* fall through */
+                _Py_FALLTHROUGH;
             case 0:
                 if (do_raise(tstate, exc, cause)) {
                     assert(oparg == 0);