rust: kbuild: disambiguate zerocopy for rusttest

Starting with Rust 1.76.0, `zerocopy` was added as an (indirect)
compiler dependency [1]. In turn, this meant that the `rustc-dev`
component started including a precompiled `zerocopy` crate in the sysroot.

This makes `rusttest` fail because the compiler finds several candidates:

    error[E0464]: multiple candidates for `rmeta` dependency `zerocopy` found
      --> rust/kernel/prelude.rs:64:9
       |
    64 | pub use zerocopy::{
       |         ^^^^^^^^
       |
       = note: candidate #1: .../lib/rustlib/x86_64-unknown-linux-gnu/lib/libzerocopy-dfef4cb07ca752aa.rmeta
       = note: candidate #2: ./rust/test/libzerocopy.rlib

We cannot use `--sysroot=/dev/null` for these, thus point to the dependency
explicitly.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Antoni Boucher <[email protected]>
Cc: [email protected]
Fixes: 567621523a ("rust: zerocopy: enable support in kbuild")
Link: https://github.com/rust-lang/rust/pull/118546 [1]
Link: https://patch.msgid.link/[email protected]
[ Investigated when it started happening, reworded to add that and to
  follow our usual style and sent on behalf of Antoni, who found this
  during his work to support Rust for Linux with the GCC backend, i.e.
  with `rustc_codegen_gcc`. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

rust/Makefile

@@ -350,7 +350,7 @@ rusttestlib-pin_init: $(src)/pin-init/src/lib.rs rusttestlib-macros \
 rusttestlib-kernel: private rustc_target_flags = --extern ffi \
     --extern build_error --extern macros --extern pin_init \
     --extern bindings --extern uapi \
-    --extern zerocopy --extern zerocopy_derive
+    --extern zerocopy=$(objtree)/$(obj)/test/libzerocopy.rlib --extern zerocopy_derive
 rusttestlib-kernel: $(src)/kernel/lib.rs rusttestlib-bindings rusttestlib-uapi \
     rusttestlib-build_error rusttestlib-pin_init $(obj)/$(libmacros_name) \
     $(obj)/bindings.o rusttestlib-zerocopy rusttestlib-zerocopy_derive FORCE