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

[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.

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Commit messages:
 - 8319980: [JVMCI] libgraal should reuse Thread instances as C2 does

Changes: https://git.openjdk.org/jdk/pull/16638/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16638&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319980
  Stats: 19 lines in 1 file changed: 4 ins; 0 del; 15 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