Don't call GetOEMCP() in Universal Windows Platform builds

Don't call GetOEMCP() in Universal Windows Platform builds

It's not available [1] [2]. However we can use the intermediate CP_OEMCP
value. It can be used to compare charsets in create_sconv_object().
It won't work with comparing charsets in archive_string_default_conversion_for_read()
and archive_string_default_conversion_for_write(). current_codepage being
an actual CodePage value.

[1] https://learn.microsoft.com/en-us/uwp/win32-and-com/win32-apis#apis-from-api-ms-win-core-localization-l1-2-0dll
[2] https://learn.microsoft.com/en-us/windows/win32/api/winnls/nf-winnls-getoemcp#requirements