RFR: 8334772: Change Class::signers to an explicit field [v2]

[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 incrementally with one additional commit since the last revision:

  Reorder comment of classData to avoid misunderstanding

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20223/files
  - new: https://git.openjdk.org/jdk/pull/20223/files/86b3a248..dd62b9d2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20223&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20223&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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