RFR: 8294729: [s390] Implement nmethod entry barriers [v13]

[Tyler Steele]

On Thu, 27 Oct 2022 21:06:37 GMT, Tyler Steele <tsteele at openjdk.org> wrote:

>> This draft PR implements native method barriers on s390. When complete, this will fix the build, and bring the other benefits of [JDK-8290025](https://bugs.openjdk.org/browse/JDK-8290025) to that platform.
>
> Tyler Steele has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Clean up comments. Adjust copyright years.

Written earlier; tests just completed.

I made some changes:

- I agree that nmethod_entry_barrier was in a silly place, so I moved it to a better one. This change won't need to be reviewed.
- I have added the c2i entry barrier code which will need to be reviewed. I believe any of my reviewers would be able to do this.
- I have added a first pass for the changes requested to `g1_write_barrier_pre()`. I am not totally clear on what needs to happen, so there will probably be more to do here. Please take a look @fisk.

I've run the tests in `test/hotspot/jtreg/gc` again. They are passing, but since they passed before the change I don't believe this tells up much. Is there a better suite to run, or is this functionality not covered by any test suite? I imagine getting the GC to a particular state, then triggering a usually automatic action would be very difficult in many cases.

-------------

PR: https://git.openjdk.org/jdk/pull/10558