JDK-8365053 made a fact which is already well known to ZGC developers clear. We pull in large parts of the ZGC implementation through the access API, via `zbarrierset.inline.hpp`.

ZGC developers are well aware as touching most `.hpp` or `inline.hpp` files in `gc/z` requires rebuilding most of hotspot in incremental builds.

I propose we create a boundary between the barrier set and the implementation. The main reason being making incremental builds less painful.

I experimented with this last year, at the time I saw no real difference in full build times, nor any performance regressions from not inlining the barrier implementation into the access API.

Will reevaluate the performance implications.