rust: kbuild: clean output before running `rustdoc`

`rustdoc` can get confused when generating documentation into a folder
that contains generated files from other `rustdoc` versions.

For instance, running something like:

rustup default 1.78.0
make LLVM=1 rustdoc
rustup default 1.88.0
make LLVM=1 rustdoc

may generate errors like:

error: couldn't generate documentation: invalid template: last line expected to start with a comment
|
= note: failed to create or modify "./Documentation/output/rust/rustdoc/src-files.js"

Thus just always clean the output folder before generating the
documentation -- we are anyway regenerating it every time the `rustdoc`
target gets called, at least for the time being.

Cc: [e-mail] # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Reported-by: Daniel Almeida <[e-mail]>
Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/288089/topic/x/near/527201113
Reviewed-by: Tamir Duberstein <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Miguel Ojeda <[e-mail]>

rust/Makefile

···
 rustdoc-macros: private rustdoc_host = yes
 rustdoc-macros: private rustc_target_flags = --crate-type proc-macro \
     --extern proc_macro
-rustdoc-macros: $(src)/macros/lib.rs FORCE
+rustdoc-macros: $(src)/macros/lib.rs rustdoc-clean FORCE
 	+$(call if_changed,rustdoc)
 
 # Starting with Rust 1.82.0, skipping `-Wrustdoc::unescaped_backticks` should
 # not be needed -- see https://github.com/rust-lang/rust/pull/128307.
 rustdoc-core: private skip_flags = --edition=2021 -Wrustdoc::unescaped_backticks
 rustdoc-core: private rustc_target_flags = --edition=$(core-edition) $(core-cfgs)
-rustdoc-core: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
+rustdoc-core: $(RUST_LIB_SRC)/core/src/lib.rs rustdoc-clean FORCE
 	+$(call if_changed,rustdoc)
 
 rustdoc-compiler_builtins: $(src)/compiler_builtins.rs rustdoc-core FORCE
···
 rustdoc-pin_init_internal: private rustdoc_host = yes
 rustdoc-pin_init_internal: private rustc_target_flags = --cfg kernel \
     --extern proc_macro --crate-type proc-macro
-rustdoc-pin_init_internal: $(src)/pin-init/internal/src/lib.rs FORCE
+rustdoc-pin_init_internal: $(src)/pin-init/internal/src/lib.rs \
+    rustdoc-clean FORCE
 	+$(call if_changed,rustdoc)
 
 rustdoc-pin_init: private rustdoc_host = yes
···
     rustdoc-pin_init rustdoc-compiler_builtins $(obj)/$(libmacros_name) \
     $(obj)/bindings.o FORCE
 	+$(call if_changed,rustdoc)
+
+rustdoc-clean: FORCE
+	$(Q)rm -rf $(rustdoc_output)
 
 quiet_cmd_rustc_test_library = $(RUSTC_OR_CLIPPY_QUIET) TL $<
       cmd_rustc_test_library = \