RFR: 8372617: Save and restore stubgen stubs when using an AOT code cache [v11]

[Andrew Dinn]

> This PR adds save and restore of all generated stubs to the AOT code cache on x86 and aarch64. Other arches are modified to deal with the related generic PAI changes.
> 
> Small changes were required to the aarch64 and x86_64 generator code in order to meet two key constraints:
> 1. the first declared entry of every stub starts at the first instruction in the stub code range
> 2. all data/code cross-references from one stub to another target a declared stub entry

Andrew Dinn has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 43 commits:

 - merge
 - ensure C1 treats ThreadIdentifier::unsafe_offset() as an external address
 - configure low heap size to exercise more stub code
 - Run AOTCodeFlags test with multiple GCs
 - add missing external for aarch64
 - fix thinko in idx save restore and tweak aarch64 external addresses
 - added missing GC external addresses and speeded up address to id lookup
 - fix whitespace issue
 - remaining asmehra feedback
 - asmehra feedback
 - ... and 33 more: https://git.openjdk.org/jdk/compare/d49e29aa...ec219c0b

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Changes: https://git.openjdk.org/jdk/pull/28433/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28433&range=10
  Stats: 5660 lines in 62 files changed: 4638 ins; 446 del; 576 mod
  Patch: https://git.openjdk.org/jdk/pull/28433.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28433/head:pull/28433

PR: https://git.openjdk.org/jdk/pull/28433