[ObjCRuntime] Add missing cases for UIntPtr for determining if a type…

… is a built-in type. (#15391)

Also fix a typo for "System.nint".

Co-authored-by: Manuel de la Pena <mandel@microsoft.com>

src/ObjCRuntime/Stret.cs

@@ -138,8 +138,9 @@ public static bool ArmNeedStret (Type returnType, Generator generator)
 					case "System.UInt32":
 					case "System.Int32":
 					case "System.IntPtr":
+					case "System.UIntPtr":
 					case "System.nuint":
-					case "System.uint":
+					case "System.nint":
 						return false;
 					// floating-point types are stret
 					}
@@ -269,6 +270,7 @@ static bool IsBuiltInType (Type type, bool is_64_bits, out int type_size)
 				type_size = 8;
 				return true;
 			case "IntPtr":
+			case "UIntPtr":
 #if !NET
 			case "nfloat":
 #endif