RFR: 8294775: Shenandoah: reduce contention on _threads_in_evac [v3]

[Nick Gasson]

> The idea here is to reduce contention on the shared `_threads_in_evac` counter by splitting its state over multiple independent cache lines. Each thread hashes to one particular counter based on its `Thread*`. This helps improve throughput of concurrent evacuation where many Java threads may be attempting to update this counter on the load barrier slow path.
> 
> See this earlier thread for details and SPECjbb results: https://mail.openjdk.org/pipermail/shenandoah-dev/2022-October/017494.html
> 
> Also tested `hotspot_gc_shenandoah` on x86 and AArch64.

Nick Gasson has updated the pull request incrementally with one additional commit since the last revision:

  Put the * next to the type

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10573/files
  - new: https://git.openjdk.org/jdk/pull/10573/files/14cec5ed..09447b38

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10573&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10573&range=01-02

  Stats: 11 lines in 3 files changed: 0 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/10573.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10573/head:pull/10573

PR: https://git.openjdk.org/jdk/pull/10573