[dotnet] Ignore RuntimeIdentifier if RuntimeIdentifiers is set.

It's not clear what we're supposed to do if both RuntimeIdentifier and
RuntimeIdentifiers are set, so just handle this case as if only
RuntimeIdentifiers is set, by clearing out the RuntimeIdentifier value.

Also show a warning for this scenario.

dotnet/targets/Xamarin.Shared.Sdk.DefaultItems.targets

@@ -53,6 +53,12 @@
     <IntermediateOutputPath>$(IntermediateOutputPath)$(RuntimeIdentifier)\</IntermediateOutputPath>
     <OutputPath>$(OutputPath)$(RuntimeIdentifier)\</OutputPath>
   </PropertyGroup>
+  <PropertyGroup Condition="'$(RuntimeIdentifiers)' != '' And '$(RuntimeIdentifier)' != '' ">
+    <!-- Check if both RuntimeIdentifier and RuntimeIdentifiers are set, in which case we clear RuntimeIdentifier -->
+    <!-- Also set a variable that says if this happens, so that we can show a warning later -->
+    <_RuntimeIdentifiersClashMessage>Both RuntimeIdentifier and RuntimeIdentifiers are set. The value for RuntimeIdentifier will be ignored.</_RuntimeIdentifiersClashMessage>
+    <!-- Clear RuntimeIdentifier -->
+    <RuntimeIdentifier />
   </PropertyGroup>
   <!-- This is a variation of https://github.com/dotnet/sdk/blob/873d79d861cbd001488414b9875e53acbeaed890/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.RuntimeIdentifierInference.targets#L68-L96 -->
   <!-- This doesn't cover every single possibility (in particular it does not handle ARMv7s, either as a thin or fat option, and the same for ARMv7k), but that can be done by passing /p:TargetArchitectures=ARMv7s to msbuild -->

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -240,6 +240,10 @@
 		</PropertyGroup>
 	</Target>
 
+	<Target Name="_WarnRuntimeIdentifiersClash" Condition="'$(_RuntimeIdentifiersClashMessage)' != ''">
+		<Warning Text="$(_RuntimeIdentifiersClashMessage)" />
+	</Target>
+
 	<!-- If we have multiple runtime identifiers, then we need to run an inner build for each of them -->
 	<Target Name="_BuildRidSpecificAppBundle"
 		DependsOnTargets="Build"