RFR: 7903494: jcstress: seqcst tests arbiters should take the lock

[Aleksey Shipilev]

Found this while peeking into the generated code: the arbiters code is not `synchronized` for sync tests, although it should match what other threads are doing. This seems to be innocuous on current platforms, because arbiter always runs "last" with the relevant volatile sychronization. This is not guaranteed to work generally, though.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jcstress/pull/140/files
 Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=140&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903494
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jcstress/pull/140.diff
  Fetch: git fetch https://git.openjdk.org/jcstress.git pull/140/head:pull/140

PR: https://git.openjdk.org/jcstress/pull/140