Auto merge of #118463 - cuviper:restore-cg_gcc-ci, r=cuviper,GuillaumeGomez

Re-enable `rustc_codegen_gcc` tests in CI

When #117947 dropped llvm-15 from CI, we neglected to copy #117313's changes to enable `rustc_codegen_gcc` testing to the new base llvm-16. This is now restored, as well as copying the setup to llvm-17 as well so we hopefully won't miss it next time.

In addition, due to case mismatch in `$extra_env` updates in `docker/run.sh`, I think it wasn't actually getting enabled before, but this should now be fixed. I also avoided the linker hack for `libgccjit.so` that was present before, because that's not needed if the version matches the base `gcc` used for linking.

r? GuillaumeGomez

Author: bors