[Build] Add per-file up-to-date check in CompileNativeAssembly

When _CompileNativeAssemblySources runs, it recompiles ALL .ll files
even if only some have changed. This is because MSBuild's target-level
Inputs/Outputs is all-or-nothing — if any .ll file is newer than any
.o file, every file gets recompiled.

Add a per-file timestamp check in RunAssembler() — if the output .o is
newer than the input .ll, that file is skipped. This saves time on
incremental CoreCLR builds where upstream generators use
CopyIfStreamChanged to preserve .ll timestamps for unchanged files.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: davidnguyen-tech

src/Xamarin.Android.Build.Tasks/Tasks/CompileNativeAssembly.cs

@@ -22,6 +22,7 @@ sealed class Config
 			public string? AssemblerPath;
 			public string? AssemblerOptions;
 			public string? InputSource;
+			public string? OutputFile;
 		}
 
 		[Required]
@@ -43,6 +44,14 @@ public override System.Threading.Tasks.Task RunTaskAsync ()
 
 		void RunAssembler (Config config)
 		{
+			if (config.OutputFile is not null && config.InputSource is not null && File.Exists (config.OutputFile)) {
+				string sourceFile = Path.Combine (WorkingDirectory, Path.GetFileName (config.InputSource));
+				if (File.Exists (sourceFile) && File.GetLastWriteTimeUtc (config.OutputFile) >= File.GetLastWriteTimeUtc (sourceFile)) {
+					LogDebugMessage ($"[LLVM llc] Skipping '{Path.GetFileName (config.InputSource)}' because '{Path.GetFileName (config.OutputFile)}' is up to date");
+					return;
+				}
+			}
+
 			var stdout_completed = new ManualResetEvent (false);
 			var stderr_completed = new ManualResetEvent (false);
 			var psi = new ProcessStartInfo () {
@@ -118,10 +127,13 @@ IEnumerable<Config> GetAssemblerConfigs ()
 				string executableDir = Path.GetDirectoryName (llcPath);
 				string executableName = MonoAndroidHelper.GetExecutablePath (executableDir, Path.GetFileName (llcPath));
 
+				string outputFilePath = Path.Combine (WorkingDirectory, sourceFile.Replace (".ll", ".o"));
+
 				yield return new Config {
 					InputSource = item.ItemSpec,
 					AssemblerPath = Path.Combine (executableDir, executableName),
 					AssemblerOptions = $"{assemblerOptions} -o={outputFile} {QuoteFileName (sourceFile)}",
+					OutputFile = outputFilePath,
 				};
 			}
 		}