Shenandoah: Enable concurrent class unloading on x86_32

[Roman Kennke]

It looks ok to me. Shared changes would need further reviewing upstream
by Erik, eventually, but feel free to bake it in sh/jdk first.

I'd probably use rdi and load the thread-base into it using get_thread()
and keep the interface cleaner. Your call.

Roman


> Prerequisites:
>  1) Shenandoah: SBSA::load_at() should save/restore registers when
>     calling SATB barrier [1]
>  2) JDK-8230765: Implement nmethod barrier for x86_32 platforms [2]
> 
> 
> (2) is upstream CR, should be reviewed here. However, there is no
> implementation there that can exercise this implementation. I would
> prefer to bake here for sometime, before upstreaming it.
> 
> 
> Combined webrev:
> http://cr.openjdk.java.net/~zgu/shenandoah/conc-class_unloading_x86_32/webrev.00/
> 
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release) on Linux with x86_32 JVM.
> 
> [1]
> https://mail.openjdk.java.net/pipermail/shenandoah-dev/2019-October/010681.html
> 
> [2] http://cr.openjdk.java.net/~zgu/JDK-8230765/webrev.00/
> 
> 
> Thanks,
> 
> -Zhengyu
>