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

[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.

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Commit messages:
 - Generalize PointerInvoke to benchmark by-ref segment return

Changes: https://git.openjdk.org/panama-foreign/pull/788/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=788&range=00
  Stats: 43 lines in 2 files changed: 28 ins; 0 del; 15 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