RFR: 8297290: Use int indices to reference CDS archived primitive mirrors

[Ioi Lam]

In CDS, when archived class metadata reference an object in the archive heap, it's usually done using an integer index, which can be passed to `HeapShared::get_root(int)` to retrieve the object. This way, we don't need to worry if the archive heap has been relocated. E.g.,


oop Klass::archived_java_mirror() {
  assert(has_archived_mirror_index(), "must have archived mirror");
  return HeapShared::get_root(_archived_mirror_index);
}

However, for the primitive mirror objects (in `Universe::_mirrors[]`, which point to the `java.lang.Class` instances of the Java primitive types such as `int`, `float`, etc), we are still using a direct oop reference.

For consistency, we should change this code to use the same int indices as the rest of the system. This will also make it simpler to optimize the layout of the CDS archive heap (e.g., in [JDK-8234679](https://bugs.openjdk.org/browse/JDK-8234679)).

Testing: passed tiers 1-4

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

Commit messages:
 - fixed spaces
 - 8297290: Simplify the storage for CDS archived primitive mirrors

Changes: https://git.openjdk.org/jdk/pull/11256/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11256&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297290
  Stats: 70 lines in 6 files changed: 42 ins; 20 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/11256.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11256/head:pull/11256

PR: https://git.openjdk.org/jdk/pull/11256