add #include guard hack

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

include/ruby/backward/cxxanyargs.hpp

@@ -1,4 +1,5 @@
-///                                                     \noop-*-C++-*-vi:ft=cpp
+#ifndef RUBY_BACKWARD_CXXANYARGS_HPP                       //-*-C++-*-vi:ft=cpp
+#define RUBY_BACKWARD_CXXANYARGS_HPP
 /// @file
 /// @author     \@shyouhei
 /// @copyright  This  file  is  a  part   of  the  programming  language  Ruby.
@@ -9,8 +10,6 @@
 ///             meant to  be a backwards  compatibility shim.  Please  stick to
 ///             C++ 98 and never use newer features, like `constexpr`.
 /// @brief      Provides old prototypes for C++ programs.
-#ifndef RUBY_BACKWARD_CXXANYARGS_HPP
-#define RUBY_BACKWARD_CXXANYARGS_HPP
 #include "ruby/3/config.h"
 #include "ruby/3/intern/class.h"
 #include "ruby/3/intern/cont.h"