Remove dependency on AppDomain from a hello world app

Saves 4% in size on a hello world.

A hello world app will bring the `AppDomain` type with it. `AppDomain`'s `ToString` is quite expensive for what it does.

Maybe this can be done cleaner, suggestions welcome.

src/libraries/System.Private.CoreLib/src/System/AppContext.cs

@@ -89,7 +89,26 @@ internal static void OnFirstChanceException(object e)
         }
 #endif
 
-        internal static event EventHandler? ProcessExit;
+        private static event EventHandler? s_processExit;
+
+        // We invoke process exit callback through an indirection.
+        // Most apps will not install a callback in the first place.
+        // To invoke the callback, we need to pass the current AppDomain.
+        // AppDomain brings lots of unnecessary dependencies into trimmed apps.
+        // The indirection is set up when the callback is installed so that
+        // only apps that install a callback have AppDomain in the closure.
+        private static Action s_invokeProcessExitCallback;
+
+        internal static void AddProcessExitCallback(EventHandler eventHandler)
+        {
+            s_processExit += eventHandler;
+            s_invokeProcessExitCallback ??= () => s_processExit?.Invoke(AppDomain.CurrentDomain, EventArgs.Empty);
+        }
+
+        internal static void RemoveProcessExitCallback(EventHandler eventHandler)
+        {
+            s_processExit -= eventHandler;
+        }
 
         internal static void OnProcessExit()
         {
@@ -99,7 +118,7 @@ internal static void OnProcessExit()
                 EventListener.DisposeOnShutdown();
             }
 
-            ProcessExit?.Invoke(AppDomain.CurrentDomain, EventArgs.Empty);
+            s_invokeProcessExitCallback();
         }
 
         /// <summary>

src/libraries/System.Private.CoreLib/src/System/AppDomain.cs

@@ -74,8 +74,8 @@ public event EventHandler<FirstChanceExceptionEventArgs>? FirstChanceException
 
         public event EventHandler? ProcessExit
         {
-            add { AppContext.ProcessExit += value; }
-            remove { AppContext.ProcessExit -= value; }
+            add { AppContext.AddProcessExitCallback(value); }
+            remove { AppContext.RemoveProcessExitCallback(value); }
         }
 
         public string ApplyPolicy(string assemblyName)