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/test/ui/missing-allocator.rs

@@ -13,14 +13,16 @@
 
 #![no_std]
 #![crate_type = "staticlib"]
-#![feature(panic_implementation, lang_items, alloc)]
+#![feature(panic_implementation, alloc_error_handler, alloc)]
 
 #[panic_implementation]
 fn panic(_: &core::panic::PanicInfo) -> ! {
     loop {}
 }
 
-#[lang = "oom"]
-fn oom() {}
+#[alloc_error_handler]
+fn oom(_: core::alloc::Layout) -> ! {
+    loop {}
+}
 
 extern crate alloc;