RFR: 8350649: Class unloading accesses/resurrects dead Java mirror after JDK-8346567

[Aleksey Shipilev]

See bug for description of the bug. 

Before [JDK-8346567](https://bugs.openjdk.org/browse/JDK-8346567), we pulled class modifiers from the native `Klass*`, and so we bypassed this trouble. But now we take modifiers out of Java mirror, and this happens during unloading, which accesses/resurrects potentially dead mirror.

I think the solution is to keep storing a cached modifiers field in `Klass` instead of relying on Java mirror being accessible. Unfortunately, this patch undoes the removal of `u2` field from `Klass` done in [JDK-8346567](https://bugs.openjdk.org/browse/JDK-8346567).

Additional testing:
 - [x] Linux x86_64 server fastdebug, original reproducer now passes
 - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`
 - [x] Linux x86_64 server fastdebug, `jdk_jfr`
 - [x] Linux x86_64 server fastdebug, `jdk_jfr` with `-XX:+UseShenandoahGC` now passes

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

Commit messages:
 - Re-introduce Klass cache
 - Fix

Changes: https://git.openjdk.org/jdk/pull/23775/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23775&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350649
  Stats: 18 lines in 3 files changed: 14 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23775.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23775/head:pull/23775

PR: https://git.openjdk.org/jdk/pull/23775