struct layout differences between debug and release builds

[wks]

(Moved from mmtk/mmtk-core#353)

Some structs in the mmtk-openjdk binding have fields that defers between debug/release builds of the C++ code of OpenJDK. Currently, we use the directive #[cfg(debug_assertions)] to make the difference. We should consider using other mechanisms so that we can have the Rust part (together with MMTk core) in debug build, and the C++ part in release build.