RFR: 8302780: Add support for vectorized arraycopy GC barriers [v2]

[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.

Erik Österlund has updated the pull request incrementally with one additional commit since the last revision:

  RISC-V fixes

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12670/files
  - new: https://git.openjdk.org/jdk/pull/12670/files/27f30b40..676fc887

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12670&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12670&range=00-01

  Stats: 61 lines in 3 files changed: 0 ins; 55 del; 6 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