RFR: 8301106 allow cds interned strings to be moved by gc

[Ioi Lam]

**Background:**

Currently, the archived java strings are mapped in the G1 "[closed archive](https://github.com/openjdk/jdk/blob/574b48c6925ebfb31345fc46c7d23aa4153f99b0/src/hotspot/share/gc/g1/heapRegionType.hpp#L80-L92)" region. This essentially pins all the strings in memory.

As a prerequisite for  ([JDK-8296263](https://bugs.openjdk.org/browse/JDK-8296263)), this PR removes the requirement of pinning the archived strings. This will allow the CDS archive heap to be mapped in garbage collectors that do not support object pinning.

**Code changes:**

- The archived strings are referenced through an objArray (`_shared_strings_array`) to keep them alive. As a result, it's no longer necessary to pin them.
- Since it's possible for the GC to move these strings, the `_shared_table` in stringTable.cpp is modified to store a 32-bit index for each archived string. This index is used to retrieve the archived string from `_shared_strings_array` at runtime.

Note that CDS has a limit on the size of archived objArrays. When there's a large number of strings, we use a two-level table. See the comments around  `_shared_strings_array` in the header file.

**Testing**

Tiers 1 - 4

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Commit messages:
 - cleanup2
 - cleanup
 - 8301106: Allow archived Java strings to be moved by the collector

Changes: https://git.openjdk.org/jdk/pull/12607/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12607&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301106
  Stats: 241 lines in 8 files changed: 128 ins; 66 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/12607.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12607/head:pull/12607

PR: https://git.openjdk.org/jdk/pull/12607