LTO tests should not require -fat-lto-objects when only bitcode is requested

In [the last sync](https://github.com/rust-lang/rustc_codegen_gcc/pull/535), we needed a [workaround](https://github.com/rust-lang/rustc_codegen_gcc/blob/98ed962c7d3eebe12c97588e61245273d265e72f/src/back/write.rs#L60-L61) to keep the tests passing in the CI.