[foreign-memaccess+abi] RFR: Add `final` to AbstractMemorySegmentImpl::asUnbounded

[Maurizio Cimadamore]

The recent changes in https://git.openjdk.org/panama-foreign/pull/775 cause some issue in a test which ensures that caller sensitive methods are declared correctly. More specifically, the test fails whenever a non-final caller sensitive method is found. The fix is to simply add `final` to `AbstractMemorySegmentImpl::asUnbounded`.

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Commit messages:
 - Add `final` to AbstractMemorySegmentImpl::asUnbounded

Changes: https://git.openjdk.org/panama-foreign/pull/784/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=784&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/784.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/784/head:pull/784

PR: https://git.openjdk.org/panama-foreign/pull/784