RFR: 8319980: [JVMCI] libgraal should reuse Thread instances as C2 does [v3]

[Tom Rodriguez]

> Reuse Threads instances for compiler threads in the same way C1/C2 do.  This has been in use for several months now.  mach5 testing is underway.

Tom Rodriguez has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge remote-tracking branch 'origin/master' into tkr-reuse-java-thread
 - Refactor thread creation
 - Simplify logic
 - 8319980: [JVMCI] libgraal should reuse Thread instances as C2 does

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16638/files
  - new: https://git.openjdk.org/jdk/pull/16638/files/8c43c7d6..aff529d8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16638&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16638&range=01-02

  Stats: 633915 lines in 1762 files changed: 97609 ins; 473471 del; 62835 mod
  Patch: https://git.openjdk.org/jdk/pull/16638.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16638/head:pull/16638

PR: https://git.openjdk.org/jdk/pull/16638