libstdc++: Implement P2918R0 "Runtime format strings II" for C++26

This adds std::runtime_format for C++26. These new overloaded functions
enhance the std::format API so that it isn't necessary to use the less
ergonomic std::vformat and std::make_format_args (which are meant to be
implementation details). This was approved in Kona 2023 for C++26.

libstdc++-v3/ChangeLog:

	* include/std/format (__format::_Runtime_format_string): Define
	new class template.
	(basic_format_string): Add non-consteval constructor for runtime
	format strings.
	(runtime_format): Define new function for C++26.
	* testsuite/std/format/runtime_format.cc: New test.