[foreign-jextract] RFR: Investigate ways to make handoff-like operation more explicit [v3]

[Maurizio Cimadamore]

> This patch replaces all references to `withOwnerThread(null)` with the new `share()`.

Maurizio Cimadamore has updated the pull request with a new target base due to a merge or a rebase. The pull request
now contains three commits:

 - Fix register calls
 - Merge branch 'foreign-jextract' into rebuilder-segments+jextract
 - 8253823: Investigate ways to make handoff-like operation more explicit (foreign-jextract edition)
   
   This patch replaces all references to `withOwnerThread(null)` with the new `share()`.

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Changes: https://git.openjdk.java.net/panama-foreign/pull/366/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=366&range=02
  Stats: 15 lines in 7 files changed: 0 ins; 0 del; 15 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/366.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/366/head:pull/366

PR: https://git.openjdk.java.net/panama-foreign/pull/366