Unicode conversions doesn't work on big endian machines currently.
The strings are stored as little endian. Use conversion macros from
endian.h to load and store the data properly.
Let's use wide char strings always as little endian. It seems that
Windows API also always expects data to be little endian, so it
makes sense to require wide char strings as little endian also.
The patches fixes transformations between UTF8 and UTF16 only, which are
used by freerdp. UTF32 transformations are not used by freerdp.
https://github.com/FreeRDP/FreeRDP/issues/2520