libstdc++: Guard dynamic_cast use in src/c++23/print.cc [PR115015]

Do not use dynamic_cast unconditionally, in case libstdc++ is built with
-fno-rtti.

libstdc++-v3/ChangeLog:

	PR libstdc++/115015
	* src/c++23/print.cc (__open_terminal(streambuf*)) [!__cpp_rtti]:
	Do not use dynamic_cast.

libstdc++-v3/src/c++23/print.cc

@@ -87,7 +87,7 @@ namespace
   void*
   __open_terminal(std::streambuf* sb)
   {
-#ifndef _GLIBCXX_USE_STDIO_PURE
+#if ! defined _GLIBCXX_USE_STDIO_PURE && defined __cpp_rtti
     using namespace __gnu_cxx;
 
     if (auto fb = dynamic_cast<stdio_sync_filebuf<char>*>(sb))