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_span/src/symbol.rs

@@ -415,6 +415,7 @@ symbols! {
         decl_macro,
         declare_lint_pass,
         decode,
+        default_alloc_error_handler,
         default_lib_allocator,
         default_type_parameter_fallback,
         default_type_params,