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).

compiler/rustc_error_messages/locales/en-US/metadata.ftl

@@ -150,12 +150,28 @@ metadata_no_multiple_global_alloc =
 metadata_prev_global_alloc =
     previous global allocator defined here
 
+metadata_no_multiple_alloc_error_handler =
+    cannot define multiple allocation error handlers
+    .label = cannot define a new allocation error handler
+
+metadata_prev_alloc_error_handler =
+    previous allocation error handler defined here
+
 metadata_conflicting_global_alloc =
     the `#[global_allocator]` in {$other_crate_name} conflicts with global allocator in: {$crate_name}
 
+metadata_conflicting_alloc_error_handler =
+    the `#[alloc_error_handler]` in {$other_crate_name} conflicts with allocation error handler in: {$crate_name}
+
 metadata_global_alloc_required =
     no global memory allocator found but one is required; link to std or add `#[global_allocator]` to a static item that implements the GlobalAlloc trait
 
+metadata_alloc_func_required =
+    `#[alloc_error_handler]` function required, but not found
+
+metadata_missing_alloc_error_handler =
+    use `#![feature(default_alloc_error_handler)]` for a default error handler
+
 metadata_no_transitive_needs_dep =
     the crate `{$crate_name}` cannot depend on a crate that needs {$needs_crate_name}, but it depends on `{$deps_crate_name}`
 

compiler/rustc_error_messages/locales/en-US/passes.ftl

@@ -367,12 +367,6 @@ passes_unknown_external_lang_item =
 passes_missing_panic_handler =
     `#[panic_handler]` function required, but not found
 
-passes_alloc_func_required =
-    `#[alloc_error_handler]` function required, but not found
-
-passes_missing_alloc_error_handler =
-    use `#![feature(default_alloc_error_handler)]` for a default error handler
-
 passes_missing_lang_item =
     language item required, but not found: `{$name}`
     .note = this can occur when a binary crate with `#![no_std]` is compiled for a target where `{$name}` is defined in the standard library