RFR: 8301106 allow cds interned strings to be moved by gc [v3]

[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

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  @dholmes-ora comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12607/files
  - new: https://git.openjdk.org/jdk/pull/12607/files/4498fe48..7b0d27e2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12607&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12607&range=01-02

  Stats: 29 lines in 4 files changed: 11 ins; 8 del; 10 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