ada: Fix Ada.Directories.Modification_Time on Windows

Before this patch, Ada.Directories.Modification_Time called
GetFileAttributesExA under the hood on Windows. That would sometimes
fail to work with files whose names were non-ASCII.

This patch replaces the call to GetFileAttributesExA with a call to
GetFileAttributesEx preceded by an encoding scheme conversion, as is
done in other functions of the run-time library. This fixes the issues
that were observed with the previous implementations.

gcc/ada/

	* adaint.c (__gnat_file_time): Fix Windows version.