[jdk16] RFR: 8260704: ParallelGC: oldgen expansion needs release-store for _end

[Kim Barrett]

Please review this change that ensures MutableSpace::_end is updated after
everything else that is relevant when expanding, by using a release_store to
perform the update.  With this change the storestore that was added by
JDK-8257999 is no longer needed.

Testing:
mach5 tier1-3, tier5

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Commit messages:
 - Move JDK-8257999 barrier to correct location

Changes: https://git.openjdk.java.net/jdk16/pull/141/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=141&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260704
  Stats: 11 lines in 2 files changed: 4 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/141.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/141/head:pull/141

PR: https://git.openjdk.java.net/jdk16/pull/141