add the Win32 platform

[mrexodia]

Hello, currently the BUILD section provides Any CPU, x86, x64 and ARM but native projects usually have the platform name Win32 instead of x86 (x86 is used for managed projects).

It's not much of a hassle because it's easy to write a custom build script, but I think it's a good addition.

[XhmikosR]

@FeodorFitsner: this is related to the ci repo, not the website one, correct? If so, close the issue please.