partially apply "rust: 1.70 compat hack"

Needed as the impl_trait_in_assoc_type feature is required to build the
Asahi GPU driver.

Fixes:

    error[E0658]: `impl Trait` in associated types is unstable
       --> /home/nst/git/linux-asahi/drivers/gpu/drm/asahi/gem.rs:240:24
        |
    240 |     type Initializer = impl PinInit<Self, Error>;
        |                        ^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: see issue #63063 <rust-lang/rust#63063> for more information
        = help: add `#![feature(impl_trait_in_assoc_type)]` to the crate attributes to enable

scripts/Makefile.build

@@ -262,7 +262,7 @@ $(obj)/%.lst: $(src)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := allocator_api,new_uninit,type_alias_impl_trait
+rust_allowed_features := allocator_api,new_uninit,type_alias_impl_trait$(shell expr $$($(RUSTC) --version | cut -d' ' -f2) \> 1.69.0 >/dev/null && echo -n ,impl_trait_in_assoc_type)
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree