P2338R4 Freestanding Library: Character primitives and the C library

[shibainuudon]

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2338r4.html

__cpp_lib_freestanding_char_traits : 202306L __cpp_lib_freestanding_charconv : 202306L __cpp_lib_freestanding_cstdlib : 202306L __cpp_lib_freestanding_cstring : 202306L __cpp_lib_freestanding_cwchar : 202306L __cpp_lib_freestanding_errc : 202306L