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).

src/test/ui/feature-gates/feature-gate-alloc-error-handler.rs

@@ -5,10 +5,12 @@
 
 use core::alloc::Layout;
 
-#[alloc_error_handler] //~ ERROR the `#[alloc_error_handler]` attribute is an experimental feature
+#[alloc_error_handler] //~ ERROR use of unstable library feature 'alloc_error_handler'
 fn oom(info: Layout) -> ! {
     loop {}
 }
 
 #[panic_handler]
-fn panic(_: &core::panic::PanicInfo) -> ! { loop {} }
+fn panic(_: &core::panic::PanicInfo) -> ! {
+    loop {}
+}