[lworld] RFR: 8371357: [lworld] Remove EnableValhalla [v6]

[Paul Hübner]

> Hi all,
> 
> This removes the `EnableValhalla` in favour of the `--enable-preview` flag. Concretely:
> * I've replaced most of the `EnableValhalla` checks with `Arguments::is_valhalla_enabled()`. 
> * Some checks were redundant and could be removed entirely.
> * I've made the `EnableValhalla` flag obsolete.
> * Some tests had to be updated.
> 
> This greatly changes the semantics of tests. I've refined some test groups to make it easier.
> 
> Testing: tiers 1-4.

Paul Hübner has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 26 commits:

 - Merge branch 'lworld' into JDK-8371357
 - Merge branch 'lworld' into JDK-8371357
 - Reviewer comments.
 - Use the new function for the Parallel changes.
 - Merge branch 'lworld' into JDK-8371357
 - Remove wrong comment.
 - Fix imports.
 - Reviewer comments.
 - Copyright.
 - Revert "Don't synchronize on Double."
   
   This reverts commit cbf849c2ce6059af2b43b3dd8cb42445eab4df2b.
 - ... and 16 more: https://git.openjdk.org/valhalla/compare/ef301b76...6eae2446

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

Changes: https://git.openjdk.org/valhalla/pull/1759/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1759&range=05
  Stats: 159 lines in 56 files changed: 59 ins; 13 del; 87 mod
  Patch: https://git.openjdk.org/valhalla/pull/1759.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1759/head:pull/1759

PR: https://git.openjdk.org/valhalla/pull/1759