[msbuild/dotnet] Use properties on the _AssembliesToAOT item group to specify output paths.

This deduplicates a little bit code to compute the output path.

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -705,7 +705,7 @@
 			Condition="'$(_SdkIsSimulator)' != 'true' And '$(_PlatformName)' != 'macOS'"
 			DependsOnTargets="_ComputeVariables"
 			Inputs="@(_AssembliesToAOT)"
-			Outputs="@(_AssembliesToAOT -> '$(_AOTOutputDirectory)%(Arch)\%(Filename)%(Extension).o')">
+			Outputs="@(_AssembliesToAOT -> '%(ObjectFile)');@(_AssembliesToAOT -> '%(LLVMFile)');">
 
 		<Error Text="The AOT compiler '$(_AOTCompiler)' does not exist." Condition="!Exists ($(_AOTCompiler))" />
 
@@ -733,12 +733,11 @@
 			SdkRoot="$(_SdkRoot)"
 			TargetFrameworkMoniker="$(_ComputedTargetFrameworkMoniker)"
 		>
-			<Output TaskParameter="ObjectFiles" ItemName="_AOTObjectFiles" />
 		</CompileNativeCode>
 
 		<ItemGroup Condition="'$(IsMacEnabled)' == 'true'">
 			<!-- Add the AOT-compiled output to the main executable -->
-			<_XamarinMainLibraries Include="@(_AOTObjectFiles)" />
+			<_XamarinMainLibraries Include="@(_AssembliesToAOT -> '%(ObjectFile)')" />
 
 			<!-- copy the aotdata files to the .app -->
 			<ResolvedFileToPublish Include="%(_AssembliesToAOT.AOTData)" >

tools/dotnet-linker/Steps/ComputeAOTArguments.cs

@@ -39,6 +39,7 @@ namespace Xamarin.Linker {
 					var llvmFile = string.Empty;
 					if ((abi & Abi.LLVM) == Abi.LLVM)
 						throw ErrorHelper.CreateError (99, $"Support for LLVM hasn't been implemented yet.");
+					var objectFile = Path.Combine (outputDirectory, arch, Path.GetFileName (input) + ".o");
 					app.GetAotArguments (asm.FullPath, abi, outputDirectory, aotAssembly, llvmFile, aotData, out var processArguments, out var aotArguments, Path.GetDirectoryName (Configuration.AOTCompiler));
 					item.Metadata.Add ("Arguments", StringUtils.FormatArguments (aotArguments));
 					item.Metadata.Add ("ProcessArguments", StringUtils.FormatArguments (processArguments));
@@ -46,6 +47,7 @@ namespace Xamarin.Linker {
 					item.Metadata.Add ("Arch", arch);
 					item.Metadata.Add ("AOTData", aotData);
 					item.Metadata.Add ("AOTAssembly", aotAssembly);
+					item.Metadata.Add ("ObjectFile", objectFile);
 				}
 
 				assembliesToAOT.Add (item);