rust: enable rustdoc's --generate-link-to-definition

In Rust 1.56.0 [1], rustdoc introduced the "jump to definition"
feature [2], i.e. the unstable flag `--generate-link-to-definition`.
It adds links to the source view of the documentation.

For instance, in the source view of `rust/kernel/sync.rs`, for this code:

    impl Default for LockClassKey {
        fn default() -> Self {
            Self::new()
        }
    }

It will add three hyperlinks:

  - `Default` points to the rendered "Trait `core::default::Default`"
    page (not the source view, since it goes to another crate, though
    this may change).

  - `LockClassKey` points to the `pub struct LockClassKey(...);` line
    in the same page, highlighting the line number.

  - `Self::new()` points to the `pub const fn new() -> Self { ... }`
    associated function, highlighting its line numbers (i.e. for the
    full function).

This makes the source view more useful and a bit closer to the experience
in e.g. the Elixir Cross Referencer [3].

I have provisionally enabled it for rust.docs.kernel.org [4] -- one can
take a look at the source view there for an example of how it looks like.

Thus enable it.

Cc: Guillaume Gomez <guillaume1.gomez@gmail.com>
Link: rust-lang/rust#84176 [1]
Link: rust-lang/rust#89095 [2]
Link: https://elixir.bootlin.com [3]
Link: https://rust.docs.kernel.org [4]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -63,6 +63,7 @@ quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $<
 	OBJTREE=$(abspath $(objtree)) \
 	$(RUSTDOC) $(if $(rustdoc_host),$(rust_common_flags),$(rust_flags)) \
 		$(rustc_target_flags) -L$(objtree)/$(obj) \
+		-Zunstable-options --generate-link-to-definition \
 		--output $(rustdoc_output) \
 		--crate-name $(subst rustdoc-,,$@) \
 		$(if $(rustdoc_host),,--sysroot=/dev/null) \