Also use _stat() on Windows to be more UTF8 friendly

[liubing]

This PR tries to fix #2977.

The advantage of using _stat() function over GetFileAttributesA() API is that:

• When SPDLOG_WCHAR_FILENAMES is not defined:
  -- If setlocale(LC_ALL, ".UTF8") is called before, then filename will be treated as UTF8 string, and this is the main use case for supporting UTF8 filenames with std::string under Windows.
  -- If setlocale(LC_ALL, ".UTF8") is not called before, then filename will be treated with the current code page (CP_ACP), just like the old behavior.
• When SPDLOG_WCHAR_FILENAMES is defined, the behavior is the same as if GetFileAttributesW() was used.

[gabime]

Thanks @liubing