rust: Suppress searching builtin sysroot

By default, if Rust is passed `--target=foo` rather than a target.json
file, it will infer a default sysroot if that component is installed. As
the proposed aarch64 support [1] uses `aarch64-unknown-none` rather than a
target.json file, this is needed [2] to prevent rustc from being confused
between the custom kernel sysroot and the pre-installed one.

[ Miguel: Applied Boqun's extra case (for `rusttest`) and reworded to add
  links to the arm64 patch series discussion. In addition, fixed the
  `rustdoc` target too (which requires a conditional since `cmd_rustdoc`
  is also used for host crates like `macros`). ]

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/rust-for-linux/20231020155056.3495121-1-Jamie.Cunliffe@arm.com/ [1]
Link: https://lore.kernel.org/rust-for-linux/CAGSQo01pOixiPXkW867h4vPUaAjtKtHGKhkV-rpifJvKxAf4Ww@mail.gmail.com/ [2]
Link: https://lore.kernel.org/r/20231031201752.1189213-1-mmaurer@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -78,6 +78,7 @@ quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $<
 		$(rustc_target_flags) -L$(objtree)/$(obj) \
 		--output $(rustdoc_output) \
 		--crate-name $(subst rustdoc-,,$@) \
+		$(if $(rustdoc_host),,--sysroot=/dev/null) \
 		@$(objtree)/include/generated/rustc_cfg $<
 
 # The `html_logo_url` and `html_favicon_url` forms of the `doc` attribute
@@ -178,6 +179,7 @@ quiet_cmd_rustdoc_test_kernel = RUSTDOC TK $<
 		--extern build_error --extern macros \
 		--extern bindings --extern uapi \
 		--no-run --crate-name kernel -Zunstable-options \
+		--sysroot=/dev/null \
 		--test-builder $(objtree)/scripts/rustdoc_test_builder \
 		$< $(rustdoc_test_kernel_quiet); \
 	$(objtree)/scripts/rustdoc_test_gen
@@ -404,6 +406,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
 		--emit=metadata=$(dir $@)$(patsubst %.o,lib%.rmeta,$(notdir $@)) \
 		--crate-type rlib -L$(objtree)/$(obj) \
 		--crate-name $(patsubst %.o,%,$(notdir $@)) $< \
+		--sysroot=/dev/null \
 	$(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@)
 
 rust-analyzer:

scripts/Makefile.build

@@ -275,6 +275,7 @@ rust_common_cmd = \
 	--extern alloc --extern kernel \
 	--crate-type rlib -L $(objtree)/rust/ \
 	--crate-name $(basename $(notdir $@)) \
+	--sysroot=/dev/null \
 	--out-dir $(dir $@) --emit=dep-info=$(depfile)
 
 # `--emit=obj`, `--emit=asm` and `--emit=llvm-ir` imply a single codegen unit