Steps to reproduce:
1. Create a new Unity project
2. Build WSA player with Universal 10 SDK, Target: Mobile using IL2CPP scripting backend
3. Open VS solution
4. Deploy project on Windows Phone 10 device using Debug/ARM configuration
Result: Build fails and deploy isn't even started, error: Compiling Desktop applications for the ARM platform is not supported

Reproduced with: 5.5.0a5, 5.5.0f3, 5.6.0b1

Not reproducible with: 5.4.3f1, 5.5.0a2

Regression introduced in 5.5.0a3 - 5.5.0a5

Full output:
1>------ Build started: Project: Il2CppOutputProject, Configuration: Debug ARM ------
1> Building GameAssembly.dll with MsvcWinRtToolChain.
1> Output directory: C:\Users\WPMinion\Downloads\WSA_RAT\New folder (2)\build\bin\ARM\Debug
1> Cache directory: C:\Users\WPMinion\Downloads\WSA_RAT\New folder (2)\build\obj\il2cppOutputProject\ARM\Debug
1> ObjectFiles: 432 of which compiled: 0
1> Total compilation time: 1903 milliseconds.
1> Building MapFileParser.exe with MsvcDesktopToolChain.
1> Output directory: C:\Users\WPMinion\Downloads\WSA_RAT\New folder (2)\build\obj\il2cppOutputProject\ARM\Debug\MapFileParserCache\build
1> Cache directory: C:\Users\WPMinion\Downloads\WSA_RAT\New folder (2)\build\obj\il2cppOutputProject\ARM\Debug\MapFileParserCache
1> ObjectFiles: 16 of which compiled: 0
1> Total compilation time: 179 milliseconds.
1> Cleaned up 0 object files.
1> Encoding map file using command: C:\Users\WPMinion\Downloads\WSA_RAT\New folder (2)\build\obj\il2cppOutputProject\ARM\Debug\MapFileParserCache\build\MapFileParser.exe -format=MSVC "C:\Users\WPMinion\Downloads\WSA_RAT\New folder (2)\build\bin\ARM\Debug\GameAssembly.map" "C:\Users\WPMinion\Downloads\WSA_RAT\New folder (2)\build\bin\ARM\Debug\SymbolMap"
1> Cleaned up 1 object files.
2>------ Build started: Project: WSA_RAT, Configuration: Debug ARM ------
2> pch.cpp
2>C:\Program Files (x86)\Windows Kits\10\Include\10.0.14393.0\ucrt\corecrt.h(226): fatal error C1189: #error: Compiling Desktop applications for the ARM platform is not supported.
========== Build: 1 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
========== Deploy: 0 succeeded, 0 failed, 0 skipped ==========

Workaround:

Open "<GameName>.vcxproj" file with a text editor, find this tag:

<PreprocessorDefinitions></PreprocessorDefinitions>

And change it to this:

<PreprocessorDefinitions>_UNICODE;UNICODE;%(PreprocessorDefinitions)</PreprocessorDefinitions>

You only have to do this once in exported VS project because Unity does not overwrite this file when the project is built on top of it.