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-alloc_error_handler.stderr

@@ -0,0 +1,4 @@
+error: `#[alloc_error_handler]` function required, but not found
+
+error: aborting due to previous error
+