RFR: 8308464: Shared array class should not always be loaded in boot loader

[Calvin Cheung]

Please review this fix for using the appropriate class loader when loading a shared array class. The log output from -`Xlog:cds+unshareable=trace` is also updated to include the class loader information.

For example:
before:
`[0.203s][trace][cds,unshareable ] restore: ArrayKlassesApp`

after:
`[0.203s][trace][cds,unshareable ] restore: ArrayKlassesApp with class loader: jdk.internal.loader.ClassLoaders$AppClassLoader`

Passed tiers 1- 3 tests.

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Commit messages:
 - add assert regarding bottom_klass
 - 8308464: Shared array class should not always be loaded in boot loader

Changes: https://git.openjdk.org/jdk/pull/15421/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15421&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308464
  Stats: 37 lines in 4 files changed: 22 ins; 5 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15421.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15421/head:pull/15421

PR: https://git.openjdk.org/jdk/pull/15421