RFR: 8302780: Add support for vectorized arraycopy GC barriers

[Erik Österlund]

So far, the arraycopy stubs have performed some kind of bulk pre/post barriers for arraycopy, which have been good enough, and allowed the copying itself to be done with plain loads and stores. For generational ZGC, this approach is not good enough, and we need barriers for the actual copying, but instead don't need the pre/post barriers. To prepare the JVM for generational ZGC, we need to add an API for arraycopy barriers.

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Commit messages:
 - 8302780: Add support for vectorized arraycopy GC barriers

Changes: https://git.openjdk.org/jdk/pull/12670/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12670&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302780
  Stats: 1168 lines in 11 files changed: 859 ins; 26 del; 283 mod
  Patch: https://git.openjdk.org/jdk/pull/12670.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12670/head:pull/12670

PR: https://git.openjdk.org/jdk/pull/12670