RFR: 8352565: Add native method implementation of Reference.get() [v6]

[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.

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 10 additional commits since the last revision:

 - Merge branch 'master' into native-reference-get
 - use new waitForRefProc, some tidying
 - Merge branch 'master' into native-reference-get
 - remove timeout by using waitForReferenceProcessing
 - make ill-timed gc in non-concurrent case less likely
 - fix test package use
 - add package decl to test
 - parameterized return type of native get0
 - test native method
 - native Reference.get helper

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/24315/files
  - new: https://git.openjdk.org/jdk/pull/24315/files/48b7960c..6b4e4c76

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24315&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24315&range=04-05

  Stats: 6281 lines in 341 files changed: 3313 ins; 1807 del; 1161 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