The msbuild way of doing this would be to use the Configuration Manager (Build -> Configuration Manager) in Visual Studio to map the projects to the solution config/platform.
In the Configuration Manager:
select ARM as the Active Solution Platform and uncheck Build for all the *.UnitTests projects and ensure the other projects have Build checked.
select x86 as the Active Solution Platform check/uncheck Build based on which projects you want to build.
This will mean that whenever you build your solution for Platform=ARM, all the projects except for your *.UnitTests, will build. And similarly for Platform=x86.
You can read more about it here.
Update:
If you need more custom logic to choose projects to build, then you could create a new top level build file like:
<Project ToolsVersion="12.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup Condition="'$(Platform)' == 'ARM'">
<ARMProjects Include="abc\*.csproj" Exclude="**\*.UnitTests.csproj"/>
<ARMProjects Include="def\*.csproj" Exclude="**\*.UnitTests.csproj"/>
</ItemGroup>
<ItemGroup Condition="'$(Platform)' == 'x86'">
<!-- Create a group named X86Projects and select the projects as you need to -->
</ItemGroup>
<Target Name="Build">
<MSBuild Project="@(ARMProjects)" Targets="Build" Condition="'$(Platform)' == 'ARM'"/>
<MSBuild Project="@(X86Projects)" Targets="Build" Condition="'$(Platform)' == 'x86'"/>
</Target>
</Project>
Tweak the build targets or the projects selected according to your needs.