Implement #[alloc_error_handler]

This to-be-stable attribute is equivalent to `#[lang = "oom"]`.
It is required when using the alloc crate without the std crate.
It is called by `handle_alloc_error`, which is in turned called
by "infallible" allocations APIs such as `Vec::push`.

src/libstd/alloc.rs

@@ -125,7 +125,8 @@ fn default_alloc_error_hook(layout: Layout) {
 
 #[cfg(not(test))]
 #[doc(hidden)]
-#[lang = "oom"]
+#[cfg_attr(stage0, lang = "oom")]
+#[cfg_attr(not(stage0), alloc_error_handler)]
 #[unstable(feature = "alloc_internals", issue = "0")]
 pub fn rust_oom(layout: Layout) -> ! {
     let hook = HOOK.load(Ordering::SeqCst);