[foreign-memaccess+abi] RFR: Revert SegmentAllocator API changes

[Maurizio Cimadamore]

This patch reverts the changes to the `implSpec` of the various default methods in `SegmentAllocator`.
We have realized that the changes there are irrelevant. What matters in order to get to optimal peak performance is that `SegmentAllocator::copyArrayWithSwapIfNeeded` is force-inlined.

The segment allocator test is also reverted to its former self.

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Commit messages:
 - Revert SegmentAllocator API changes

Changes: https://git.openjdk.org/panama-foreign/pull/868/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=868&range=00
  Stats: 87 lines in 2 files changed: 32 ins; 18 del; 37 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/868.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/868/head:pull/868

PR: https://git.openjdk.org/panama-foreign/pull/868