I don't know if this is really possible or not, but you can achieve result with "Conditional compilation symbols". For example you can add ARM
only for ARM build, and build 3 separate packages: ARM, x86 and x64.
You can find a lot of articles about how to use it. This is one of them Visual Studio: Use Conditional Compilation to Control Runtime Settings for Different Deployment Scenarios.
So you just need to set up for your project for ARM configuration special symbol ARM
(like in this screenshot you see LIVE
).
After this you can use C# directives: #if, #else, #endif to hide some portion of your code for ARM build.