[foreign-memaccess+abi] RFR: Generalize PointerInvoke to benchmark by-ref segment return [v2]

[Maurizio Cimadamore]

> I've generalized an existing benchmark to test by-reference segment return in downcalls.
> Ideally, we should see scalarization of the returned segment, and no GC activity.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Fix benchmark not to let returned segments escape

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/788/files
  - new: https://git.openjdk.org/panama-foreign/pull/788/files/f5051192..1b65db66

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=788&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=788&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/788.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/788/head:pull/788

PR: https://git.openjdk.org/panama-foreign/pull/788