[objcruntime] Do not pre-compute the Selector.Name property (#12518)

it's not often required.

Delaying it's retrieval allows the linker to remove additional code (and
lower the memory for each selector) if the app's code never uses the
selector name.

```diff
@@ -2990,8 +2985,6 @@
 	{
 		private IntPtr handle;

-		private string name;
-
 		public IntPtr Handle => handle;

 		public Selector(IntPtr P_0)
@@ -3001,13 +2994,11 @@
 				ThrowHelper.ThrowArgumentException("sel", "Not a selector handle.");
 			}
 			handle = P_0;
-			name = GetName(P_0);
 		}

 		internal Selector(IntPtr P_0, bool P_1)
 		{
 			handle = P_0;
-			name = GetName(P_0);
 		}

 		public sealed override bool Equals(object? P_0)
@@ -3029,14 +3020,6 @@
 			return handle.GetHashCode();
 		}

-		internal static string GetName(IntPtr P_0)
-		{
-			return Marshal.PtrToStringAuto(sel_getName(P_0));
-		}
-
-		[DllImport("/usr/lib/libobjc.dylib")]
-		private static extern IntPtr sel_getName(IntPtr P_0);
-
 		[DllImport("/usr/lib/libobjc.dylib", EntryPoint = "sel_registerName")]
 		public static extern IntPtr GetHandle(string P_0);

```

src/ObjCRuntime/Selector.cs

@@ -42,15 +42,14 @@ public partial class Selector : IEquatable<Selector>, INativeObject {
 #endif
 
 		IntPtr handle;
-		string name;
+		string? name;
 
 		public Selector (IntPtr sel)
 		{
 			if (!sel_isMapped (sel))
 				ObjCRuntime.ThrowHelper.ThrowArgumentException (nameof (sel), "Not a selector handle.");
 
 			this.handle = sel;
-			name = GetName (sel);
 		}
 
 		// this .ctor is required, like for any INativeObject implementation
@@ -59,7 +58,6 @@ public Selector (IntPtr sel)
 		internal Selector (IntPtr handle, bool /* unused */ owns)
 		{
 			this.handle = handle;
-			name = GetName (handle);
 		}
 
 		public Selector (string name)
@@ -73,7 +71,11 @@ public IntPtr Handle {
 		}
 
 		public string Name {
-			get { return name; }
+			get {
+				if (name == null)
+					name = GetName (handle);
+				return name;
+			}
 		}
 
 		public static bool operator!= (Selector left, Selector right) {