[C++ Builder] Cannot add 64-bit windows platform

1. Open project file (*.cbproj) from a text editor.
2. Find <Platforms>, it may looks like:

<Platforms>
<Platform value="Win32">True</Platform>
</Platforms>

3. Add   <Platform value="Win64">True</Platform> 

<Platforms>
<Platform value="Win32">True</Platform>
<Platform value="Win64">True</Platform>
</Platforms>

4. Save project file.
5 Reopen the project.


[Reference] https://forums.embarcadero.com/message.jspa?messageID=823436