Deny warnings in CI

There are currently a lot of warnings printed in CI, mostly dead code.
Update CI to deny warnings.

library/compiler-builtins/.github/workflows/main.yml

@@ -1,6 +1,10 @@
 name: CI
 on: [push, pull_request]
 
+env:
+  RUSTDOCFLAGS: -Dwarnings
+  RUSTFLAGS: -Dwarnings
+
 jobs:
   test:
     name: Test

library/compiler-builtins/ci/run-docker.sh

@@ -48,6 +48,7 @@ run() {
     docker run \
            --rm \
            -e RUST_COMPILER_RT_ROOT \
+           -e RUSTFLAGS \
            -e "CARGO_TARGET_DIR=/builtins-target" \
            -v "$(pwd):/checkout:ro" \
            -w /checkout \