RFR: 8349146: [REDO] Implement a better allocator for downcalls [v8]

[Per Minborg]

On Fri, 2 May 2025 11:13:27 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR is based on the work of @mernst-github and aims to implement an _internal_ thread-local 'stack' allocator, which works like a dynamically sized arena, but with reset functionality to reset the allocated size back to a certain level. The underlying memory could stay around between calls, which could improve performance.
>> 
>> Re-allocated segments are not zeroed between allocations.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Cleanup and only create CleanupAction once per thread

The performance before this PR can be seen in the "confined" benchmarks above. In those benchmarks, a regular `Arena.ofConfined` is created upon every invocation.

I will integrate this PR now @wenshao . Can you summarize your proposed changes below as It was a bit unclear to me what you meant. I can create a separate PR with those changes later.

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

PR Comment: https://git.openjdk.org/jdk/pull/24829#issuecomment-2847309534
PR Comment: https://git.openjdk.org/jdk/pull/24829#issuecomment-2847314767