[lilliput-jdk17u:lilliput] RFR: 8303450: [Lilliput/JDK17] Runtime flag to enable Lilliput [v3]

[Roman Kennke]

> This change introduces an experimental runtime flag -XX:[+/-]UseCompactObjectHeaders which can be used to disable Lilliput. The flag guards all the relevant places that directly affect the object layout and the accesses to Klass* etc. The change also restores the previous behaviour when -UseCompactObjectHeaders is selected.
> 
> One special difficulty is that CDS archives that are generated with compact object headers cannot currently be read with old-style headers, and vice versa. I work around this by disabling CDS when UseCompactObjectHeaders is set to non-default. This makes a few tests fail, but should otherwise be harmless. (In theory, we could implement parsing archives and loading objects with the correct header layout, but ... life's too short ;-) )
> 
> Testing:
>  - [ ] tier1 (x86_64, x86_32, aarch64) +UseCompactObjectHeaders
>  - [ ] tier2 (x86_64, x86_32, aarch64) +UseCompactObjectHeaders
>  - [ ] tier1 (x86_64, x86_32, aarch64) -UseCompactObjectHeaders
>  - [ ] tier2 (x86_64, x86_32, aarch64) -UseCompactObjectHeaders

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Remove misplaced assert

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

Changes:
  - all: https://git.openjdk.org/lilliput-jdk17u/pull/8/files
  - new: https://git.openjdk.org/lilliput-jdk17u/pull/8/files/dc485a98..b64969ba

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=lilliput-jdk17u&pr=8&range=02
 - incr: https://webrevs.openjdk.org/?repo=lilliput-jdk17u&pr=8&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/lilliput-jdk17u/pull/8.diff
  Fetch: git fetch https://git.openjdk.org/lilliput-jdk17u pull/8/head:pull/8

PR: https://git.openjdk.org/lilliput-jdk17u/pull/8