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.

ARAlice Ryhl committed 2 years ago