RFR: 8352565: Add native method implementation of Reference.get()

[Kim Barrett]

Please review this change which adds a native method providing the
implementation of Reference::get.  Referece::get is an intrinsic candidate, so
this native method implementation is only used when the intrinsic is not.

Currently there is intrinsic support by the interpreter, C1, C2, and graal,
which are always used.  With this change we can later remove all the
per-platform interpreter intrinsic implementations, and might also remove the
C1 intrinsic implementation.

Testing:
(1) mach5 tier1-6 normal (so using all the existing intrinsics).
(2) mach5 tier1-6 with interpreter and C1 Reference::get intrinsics disabled.

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Commit messages:
 - test native method
 - native Reference.get helper

Changes: https://git.openjdk.org/jdk/pull/24315/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24315&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352565
  Stats: 203 lines in 5 files changed: 199 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/24315.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24315/head:pull/24315

PR: https://git.openjdk.org/jdk/pull/24315