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

@@ -3,7 +3,7 @@
 use gccjit::{FunctionType, GlobalKind, ToRValue};
 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_middle::bug;
 use rustc_middle::ty::TyCtxt;
@@ -127,4 +127,9 @@ pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut GccContext, _module_nam
     let value = tcx.sess.opts.unstable_opts.oom.should_panic();
     let value = context.new_rvalue_from_int(i8, value as i32);
     global.global_set_initializer_rvalue(value);
+
+    let name = NO_ALLOC_SHIM_IS_UNSTABLE.to_string();
+    let global = context.new_global(None, GlobalKind::Exported, i8, name);
+    let value = context.new_rvalue_from_int(i8, 0);
+    global.global_set_initializer_rvalue(value);
 }