rust: alloc: Add global_alloc_ty lang_item

Minimal sync to rust-1.78, still builds with rust-1.76.

Signed-off-by: Janne Grunau <j@jannau.net>

Author: jannau

rust/alloc/alloc.rs

@@ -52,6 +52,7 @@ extern "Rust" {
 #[unstable(feature = "allocator_api", issue = "32838")]
 #[derive(Copy, Clone, Default, Debug)]
 #[cfg(not(test))]
+#[cfg_attr(version("1.78"), lang = "global_alloc_ty")]
 pub struct Global;
 
 #[cfg(test)]