RFR: 8334772: Change Class::signers to an explicit field [v3]

[Chen Liang]

> `Class` has 2 VM-injected fields that can be made explicit: `Object[] signers` and `ProtectionDomain protectionDomain`. We make the signers field explicit. (The ProtectionDomain can be revisited when SecurityManager is removed, as SecurityManager is accessing it via JNI as well.)
> 
> Migrate the JNI code to Java. The getter previously had a redundant primitive type check, which is dropped in the migrated Java code. The `Object[] getSigners` is no longer `native`, thus requiring a CSR record. Reviewers please help review the associated CSR.

Chen Liang 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 three additional commits since the last revision:

 - Merge branch 'master' of https://github.com/openjdk/jdk into feature/class-signers
 - Reorder comment of classData to avoid misunderstanding
 - 8334772: Change Class::signers to an explicit field

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20223/files
  - new: https://git.openjdk.org/jdk/pull/20223/files/dd62b9d2..5d742e34

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20223&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20223&range=01-02

  Stats: 779 lines in 28 files changed: 676 ins; 29 del; 74 mod
  Patch: https://git.openjdk.org/jdk/pull/20223.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20223/head:pull/20223

PR: https://git.openjdk.org/jdk/pull/20223