Consider putting OS-specific platform stuff in OS subfolders

Files directly within [Platform](https://github.com/beyond-all-reason/spring/blob/master/rts/System/Platform/) sometimes contain inline OS checks like `#ifdef _WIN32`, perhaps move some of these to the appropriate subfolders (e.g. [Platform/Win](https://github.com/beyond-all-reason/spring/tree/master/rts/System/Platform/Win))?

Start by reviewing the files directly inside rts/System/Platform/ and comparing them with the OS-specific folders, especially rts/System/Platform/Win/. Search for inline _WIN32 checks and determine which platform-specific code is suitable for relocation. Done means the appropriate OS-specific logic is organized in the relevant subfolders without changing cross-platform behavior.

Written by the indexing model from the issue text.