bpo-43445: Add frozen modules to sys.stdlib_module_names (GH-24798)

Add frozen modules to sys.stdlib_module_names. For example, add
"_frozen_importlib" and "_frozen_importlib_external" names.

Add "list_frozen" command to Programs/_testembed.
diff --git a/Python/stdlib_module_names.h b/Python/stdlib_module_names.h
index 01aa675..33fba38 100644
--- a/Python/stdlib_module_names.h
+++ b/Python/stdlib_module_names.h
@@ -32,6 +32,8 @@ static const char* _Py_stdlib_module_names[] = {
 "_dbm",
 "_decimal",
 "_elementtree",
+"_frozen_importlib",
+"_frozen_importlib_external",
 "_functools",
 "_gdbm",
 "_hashlib",