rust: cleaner rust target-applies-to-host fix

[jameshilliard]

Looks like we can set this without passing a command line option.

[heitbaum]

@jameshilliard - sorry to hijack your PR - whilst you are revisiting rust could you look at how to teach rust to use $HOSTCC instead of cc?

Our https://github.com/LibreELEC/LibreELEC.tv/blob/master/tools/docker/focal/Dockerfile does not include cc (I have updated the other places where cc Was being used inadvertently - but a bit stuck on fixing rust.)

I tried a few things like below with no success yet.

-  cargo build \
+  cargo update
+  export CC=/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/toolchain
/bin/x86_64-libreelec-linux-gnu-gcc
+  RUST_LOG=debug cargo build \
     ${CARGO_Z_TARGET_APPLIES_TO_HOST} \
-    --release \
+    --all-features \
     --locked \
-    --all-features
+    --release \
+    --verbose

rust Error

error: linker `cc` not found
  |
  = note: No such file or directory (os error 2)

error: could not compile `log` due to previous error

Caused by:
  process didn't exit successfully: `rustc --crate-name build_script_build /var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/rust-1.57.0/cargo/registry/src/github.com-1ecc6299db9ec823/log-0.4.14/build.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=0 -C debug-assertions=off --cfg 'feature="std"' -C metadata=912dbdd2dbc7a629 -C extra-filename=-912dbdd2dbc7a629 --out-dir /var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/bottom-0.6.6/.x86_64-libreelec-linux-gnu/release/build/log-912dbdd2dbc7a629 -L dependency=/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/bottom-0.6.6/.x86_64-libreelec-linux-gnu/release/deps --cap-lints allow` (exit status: 1)
warning: build failed, waiting for other jobs to finish...
error: build failed
FAILURE: s/build bottom during make_target (package.mk)
*********** FAILED COMMAND ***********
RUST_LOG=debug cargo build ${CARGO_Z_TARGET_APPLIES_TO_HOST} --all-features --locked --release --verbose
**************************************

[jameshilliard]

Maybe needs to be set in the host-config?

You probably need to do something like this to enable it in stable:

__CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS="nightly"
CARGO_UNSTABLE_TARGET_APPLIES_TO_HOST="true"
CARGO_UNSTABLE_HOST_CONFIG="true"