RFR: 8271215: Fix data races in G1PeriodicGCTask

[Kim Barrett]

Please review this small change to G1PeriodicGCTask, eliminating unlikely
but possible data races with a concurrently running GC pause.  An STS-joiner
is used to ensure there isn't a GC pause while doing the checks to decide
whether to perform a periodic GC.

Testing:
mach5 tier1

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Commit messages:
 - use STS-joiner to prevent GC safepoint concurrency

Changes: https://git.openjdk.java.net/jdk/pull/4899/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4899&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271215
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4899.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4899/head:pull/4899

PR: https://git.openjdk.java.net/jdk/pull/4899