Prevent insta-stable no alloc shim support

You will need to add the following as replacement for the old __rust_*
definitions when not using the alloc shim.

    #[no_mangle]
    static __rust_no_alloc_shim_is_unstable: u8 = 0;

src/allocator.rs

@@ -5,7 +5,7 @@
 
 use rustc_ast::expand::allocator::{
     alloc_error_handler_name, default_fn_name, global_fn_name, AllocatorKind, AllocatorTy,
-    ALLOCATOR_METHODS,
+    ALLOCATOR_METHODS, NO_ALLOC_SHIM_IS_UNSTABLE,
 };
 use rustc_codegen_ssa::base::allocator_kind_for_codegen;
 use rustc_session::config::OomStrategy;
@@ -94,4 +94,11 @@ fn codegen_inner(
     let val = oom_strategy.should_panic();
     data_ctx.define(Box::new([val]));
     module.define_data(data_id, &data_ctx).unwrap();
+
+    let data_id =
+        module.declare_data(NO_ALLOC_SHIM_IS_UNSTABLE, Linkage::Export, false, false).unwrap();
+    let mut data_ctx = DataContext::new();
+    data_ctx.set_align(1);
+    data_ctx.define(Box::new([0]));
+    module.define_data(data_id, &data_ctx).unwrap();
 }