RFR: 8275051: Shenandoah: Enforce ordering of requested gc cause and gc request flag

[Zhengyu Gu]

Although setting gc request is under _gc_waiters_lock, but read side (run_service()) does not take the lock. We need to enforce following ordering, so that read side sees latest requested gc cause when the flag is set.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - Merge branch 'master' into JDK-8275051-gc_req_order
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/5898/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5898&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275051
  Stats: 10 lines in 1 file changed: 5 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5898.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5898/head:pull/5898

PR: https://git.openjdk.java.net/jdk/pull/5898