RFR: 8257817: Shenandoah: Don't race with conc-weak-in-progress flag in weak-LRB

[Zhengyu Gu]

After concurrent weak root processing, it should perform handshake first to ensure there are no dirty loads (loads have yet processed by barriers) in Java thread, before it resets conc-weak-root-in-progress flag.

- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8257817

Changes: https://git.openjdk.java.net/jdk/pull/1673/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1673&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257817
  Stats: 7 lines in 1 file changed: 4 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1673.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1673/head:pull/1673

PR: https://git.openjdk.java.net/jdk/pull/1673