Implement Make handle_alloc_error default to panic (for no_std + liballoc)

Related: https://github.com/rust-lang/rust/issues/66741

Guarded with `#![feature(default_alloc_error_handler)]` a default
`alloc_error_handler` is called, if a custom allocator is used and no
other custom `#[alloc_error_handler]` is defined.

The panic message does not contain the size anymore, because it would
pull in the fmt machinery, which would blow up the code size
significantly.

compiler/rustc_feature/src/active.rs

@@ -593,6 +593,9 @@ declare_features! (
     /// Allows to use the `#[cmse_nonsecure_entry]` attribute.
     (active, cmse_nonsecure_entry, "1.48.0", Some(75835), None),
 
+    /// Allows rustc to inject a default alloc_error_handler
+    (active, default_alloc_error_handler, "1.48.0", Some(66741), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------