Add deployment-target --print flag for Apple targets

compiler/rustc_driver_impl/src/lib.rs

@@ -745,6 +745,22 @@ fn print_crate_info(
                     }
                 }
             }
+            DeploymentTarget => {
+                use rustc_target::spec::current_apple_deployment_target;
+
+                if sess.target.is_like_osx {
+                    safe_println!(
+                        "deployment_target={}",
+                        current_apple_deployment_target(&sess.target)
+                            .expect("unknown Apple target OS")
+                    )
+                } else {
+                    early_error(
+                        ErrorOutputType::default(),
+                        "only Apple targets currently support deployment version info",
+                    )
+                }
+            }
         }
     }
     Compilation::Stop