RFR: 8349146: [REDO] Implement a better allocator for downcalls [v3]

[Per Minborg]

> 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:

  Convert to junit tests

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24829/files
  - new: https://git.openjdk.org/jdk/pull/24829/files/1e9d0460..101882d5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24829&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24829&range=01-02

  Stats: 30 lines in 2 files changed: 3 ins; 2 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/24829.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24829/head:pull/24829

PR: https://git.openjdk.org/jdk/pull/24829