Don't abort if the runtime is run twice.

This changes an `assert_once_ever!` assertion to just a plain old assertion
around an atomic boolean to ensure that one particular runtime doesn't attempt
to exit twice.

Closes #9739

src/test/run-pass/rt-run-twice.rs

@@ -0,0 +1,26 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// xfail-fast windows uses a different test runner
+
+use std::rt;
+
+#[start]
+fn start(argc: int, argv: **u8) -> int {
+    do rt::start(argc, argv) {
+        println("First invocation");
+    };
+
+    do rt::start(argc, argv) {
+        println("Second invocation");
+    };
+
+    0
+}