Rewrite implementation of #[alloc_error_handler]

The new implementation doesn't use weak lang items and instead changes
`#[alloc_error_handler]` to an attribute macro just like
`#[global_allocator]`.

The attribute will generate the `__rg_oom` function which is called by
the compiler-generated `__rust_alloc_error_handler`. If no `__rg_oom`
function is defined in any crate then the compiler shim will call
`__rdl_oom` in the alloc crate which will simply panic.

This also fixes link errors with `-C link-dead-code` with
`default_alloc_error_handler`: `__rg_oom` was previously defined in the
alloc crate and would attempt to reference the `oom` lang item, even if
it didn't exist. This worked as long as `__rg_oom` was excluded from
linking since it was not called.

This is a prerequisite for the stabilization of
`default_alloc_error_handler` (#102318).

library/core/src/macros/mod.rs

@@ -1511,6 +1511,17 @@ pub(crate) mod builtin {
         /* compiler built-in */
     }
 
+    /// Attribute macro applied to a function to register it as a handler for allocation failure.
+    ///
+    /// See also [`std::alloc::handle_alloc_error`](../../../std/alloc/fn.handle_alloc_error.html).
+    #[cfg(not(bootstrap))]
+    #[unstable(feature = "alloc_error_handler", issue = "51540")]
+    #[allow_internal_unstable(rustc_attrs)]
+    #[rustc_builtin_macro]
+    pub macro alloc_error_handler($item:item) {
+        /* compiler built-in */
+    }
+
     /// Keeps the item it's applied to if the passed path is accessible, and removes it otherwise.
     #[unstable(
         feature = "cfg_accessible",