RFR: 8334772: Change Class::signers to an explicit field

[Chen Liang]

On Wed, 17 Jul 2024 19:47:44 GMT, Chen Liang <liach at openjdk.org> wrote:

> `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.

The `native` flag is not rendered in API spec, so indeed we can drop without a CSR.

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PR Comment: https://git.openjdk.org/jdk/pull/20223#issuecomment-2236359362