[foreign-memaccess+abi] RFR: Revert SegmentAllocator API changes [v2]

[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.

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

  Address review comments

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/868/files
  - new: https://git.openjdk.org/panama-foreign/pull/868/files/857006d2..eb4c37ad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=868&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=868&range=00-01

  Stats: 6 lines in 2 files changed: 0 ins; 3 del; 3 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