[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 adds 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 generators like CopyIfStreamChanged 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 && 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,
 				};
 			}
 		}