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.