ggml-backend : make path_str compatible with C++20

[ctrysbita]

#12144 breaks the compatibility with C++20

In C++20, fs::path::u8string() return type changed from std::string (C++17) to std::u8string (C++20+). This PR modifies the path_str function to use conditional compilation to detect __cpp_lib_char8_t (a standard feature flag introduced in C++20):

• When compiled with C++20+, convert std::u8string to std::string
• When compiled with C++17, use the returned std::string directly

[slaren]

@ggerganov I am not sure what are the guidelines for modifying this file, I believe that until now it has been generated automatically from the git commits. Should this change be merged as is?

[ggerganov]

Either way is fine - the gen-authors.sh script will re-generate the file.

I'll run it after this gets in.