rust: workaround rustdoc target modifiers bug

Starting with Rust 1.88.0 (released 2025-06-26), `rustdoc` complains
about a target modifier mismatch in configurations where `-Zfixed-x18`
is passed:

    error: mixing `-Zfixed-x18` will cause an ABI mismatch in crate `rust_out`
      |
      = help: the `-Zfixed-x18` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
      = note: unset `-Zfixed-x18` in this crate is incompatible with `-Zfixed-x18=` in dependency `core`
      = help: set `-Zfixed-x18=` in this crate or unset `-Zfixed-x18` in `core`
      = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=fixed-x18` to silence this error

The reason is that `rustdoc` was not passing the target modifiers when
configuring the session options, and thus it would report a mismatch
that did not exist as soon as a target modifier is used in a dependency.

We did not notice it in the kernel until now because `-Zfixed-x18` has
been a target modifier only since 1.88.0 (and it is the only one we use
so far).

The issue has been reported upstream [1] and a fix has been submitted
[2], including a test similar to the kernel case.

Meanwhile, conditionally pass `-Cunsafe-allow-abi-mismatch=fixed-x18`
to workaround the issue on our side.

Reported-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Closes: https://lore.kernel.org/rust-for-linux/36cdc798-524f-4910-8b77-d7b9fac08d77@oss.qualcomm.com/
Link: rust-lang/rust#144521 [1]
Link: rust-lang/rust#144523 [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/Makefile

@@ -74,6 +74,7 @@ quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $<
 		-Zunstable-options --generate-link-to-definition \
 		--output $(rustdoc_output) \
 		--crate-name $(subst rustdoc-,,$@) \
+		$(if $(call rustc-min-version,108800),-Cunsafe-allow-abi-mismatch=fixed-x18) \
 		$(if $(rustdoc_host),,--sysroot=/dev/null) \
 		@$(objtree)/include/generated/rustc_cfg $<