PR CI (x86_64-gnu-llvm-17) does not run codegen tests

[nikic]

https://github.com/rust-lang/rust/blob/master/src/ci/docker/scripts/x86_64-gnu-llvm.sh passes --skip tests/codegen if ENABLE_GCC_CODEGEN is enabled. The tests are disabled when using the LLVM backend as well.

[nikic]

This is less bad than I initially thought because ENABLE_GCC_CODEGEN is only enabled in PR CI, so we still at least check this in auto CI.