kbuild: fix issues with rustc-option

Fix a few different compiler errors that cause rustc-option to give
wrong results.

If KBUILD_RUSTFLAGS or the flags being tested contain any -Z flags, then
the error below is generated. The RUSTC_BOOTSTRAP environment variable
is added to fix this error.

	error: the option `Z` is only accepted on the nightly compiler
	help: consider switching to a nightly toolchain: `rustup default nightly`
	note: selecting a toolchain with `+toolchain` arguments require a rustup proxy;
	      see <https://rust-lang.github.io/rustup/concepts/index.html>
	note: for more information about Rust's stability policy, see
	      <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>
	error: 1 nightly option were parsed

Note that RUSTC_BOOTSTRAP is also defined in the top-level Makefile,
but Make-exported variables are unfortunately *not* inherited. That said,
this is changing as of commit 98da874c4303 ("[SV 10593] Export variables
to $(shell ...) commands"), which is part of Make 4.4.

The probe may also fail with the error message below. To fix it,
the /dev/null argument is replaced with a file containing the crate
attribute #![no_core]. The #![no_core] attribute ensures that rustc does
not look for the standard library. It's not possible to instead supply
a standard library (i.e. `core`) to rustc, as we need `rustc-option`
before the Rust standard library is compiled.

	error[E0463]: can't find crate for `std`
	  |
	  = note: the `aarch64-unknown-none` target may not be installed
	  = help: consider downloading the target with `rustup target add aarch64-unknown-none`
	  = help: consider building the standard library from source with `cargo build -Zbuild-std`

The -o and --out-dir parameters are altered to fix this warning:

	warning: ignoring --out-dir flag due to -o flag

The --sysroot flag is provided as we would otherwise require it to be
present in KBUILD_RUSTFLAGS. The --emit=obj flag is used to write the
resulting object file to /dev/null instead of writing it to a file
in $(TMPOUT).

I verified that the Kconfig version of rustc-option doesn't have the
same issues.

Fixes: c4229743 ("kbuild: rust: Define probing macros for rustc")
Co-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20241009-rustc-option-bootstrap-v3-1-5fa0d520efba@google.com
[ Reworded as discussed in the list. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

scripts/Makefile.compiler

@@ -73,8 +73,11 @@ ld-option = $(call try-run, $(LD) $(KBUILD_LDFLAGS) $(1) -v,$(1),$(2),$(3))
 
 # __rustc-option
 # Usage: MY_RUSTFLAGS += $(call __rustc-option,$(RUSTC),$(MY_RUSTFLAGS),-Cinstrument-coverage,-Zinstrument-coverage)
+# TODO: remove RUSTC_BOOTSTRAP=1 when we raise the minimum GNU Make version to 4.4
 __rustc-option = $(call try-run,\
-	$(1) $(2) $(3) --crate-type=rlib /dev/null --out-dir=$$TMPOUT -o "$$TMP",$(3),$(4))
+	echo '#![allow(missing_docs)]#![feature(no_core)]#![no_core]' | RUSTC_BOOTSTRAP=1\
+	$(1) --sysroot=/dev/null $(filter-out --sysroot=/dev/null,$(2)) $(3)\
+	--crate-type=rlib --out-dir=$(TMPOUT) --emit=obj=- - >/dev/null,$(3),$(4))
 
 # rustc-option
 # Usage: rustflags-y += $(call rustc-option,-Cinstrument-coverage,-Zinstrument-coverage)