RFR: 8254315: Shenandoah: Concurrent weak reference processing

[Roman Kennke]

8254315: Shenandoah: Concurrent weak reference processing

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Commit messages:
 - Merge branch 'master' into shenandoah-concurrent-weakrefs
 - Add Oracle copyright header to shenandoahReferenceProcessor.[hc]pp due to its structural origins from its ZGC couterparts
 - Relax assert in reference processor to account no LRB in passive mode
 - Aarch64 support for concurrent weak references/extended native barriers
 - Perform reference-processing during full-GC and degenerated-GC
 - Relax during-evacuation verification to account for Reference referents that have not yet been cleared
 - Apply LRB when draining ref-proc discovered lists
 - Install softref policy at init-mark pause, not at conc-mark
 - Use native-LRBs for Reference.get() intrinsics
 - Implement correct strong and final marking; Fix liveness counting
 - ... and 18 more: https://git.openjdk.java.net/jdk/compare/c9d0407e...610cd75a

Changes: https://git.openjdk.java.net/jdk/pull/505/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=505&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254315
  Stats: 2278 lines in 52 files changed: 1534 ins; 565 del; 179 mod
  Patch: https://git.openjdk.java.net/jdk/pull/505.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/505/head:pull/505

PR: https://git.openjdk.java.net/jdk/pull/505