Add iOS/tvOS/Android to AdditionalBuildTargetFrameworks

Saw these missing while reviewing dotnet#37944.
As far as I can see this is only used when building inside VS but still good to add for consistency.

src/libraries/Directory.Build.props

@@ -17,7 +17,7 @@
     <RuntimeGraph>$(LibrariesProjectRoot)OSGroups.json</RuntimeGraph>
     <BuildTargetFramework>$(NetCoreAppCurrent)</BuildTargetFramework>
     <ShouldUnsetParentConfigurationAndPlatform>false</ShouldUnsetParentConfigurationAndPlatform>
-    <AdditionalBuildTargetFrameworks Condition="'$(BuildingInsideVisualStudio)' == 'true'">$(AdditionalBuildTargetFrameworks);$(NetCoreAppCurrent)-Windows_NT;$(NetCoreAppCurrent)-Unix;$(NetCoreAppCurrent)-Browser;$(NetCoreAppCurrent)-OSX;$(NetCoreAppCurrent)-Linux;$(NetCoreAppCurrent)-NetBSD;$(NetCoreAppCurrent)-FreeBSD;$(NetCoreAppCurrent)-illumos;$(NetCoreAppCurrent)-Solaris</AdditionalBuildTargetFrameworks>
+    <AdditionalBuildTargetFrameworks Condition="'$(BuildingInsideVisualStudio)' == 'true'">$(AdditionalBuildTargetFrameworks);$(NetCoreAppCurrent)-Windows_NT;$(NetCoreAppCurrent)-Unix;$(NetCoreAppCurrent)-Browser;$(NetCoreAppCurrent)-OSX;$(NetCoreAppCurrent)-iOS;$(NetCoreAppCurrent)-tvOS;$(NetCoreAppCurrent)-Android;$(NetCoreAppCurrent)-Linux;$(NetCoreAppCurrent)-NetBSD;$(NetCoreAppCurrent)-FreeBSD;$(NetCoreAppCurrent)-illumos;$(NetCoreAppCurrent)-Solaris</AdditionalBuildTargetFrameworks>
     <!-- Remove once is fixed: https://github.com/dotnet/roslyn/issues/42344 -->
     <GeneratePlatformNotSupportedAssemblyHeaderFile>$(RepositoryEngineeringDir)LicenseHeader.txt</GeneratePlatformNotSupportedAssemblyHeaderFile>
   </PropertyGroup>