[foreign-memaccess+abi] RFR: Add missing @implSpec to AddressLayout

[Maurizio Cimadamore]

As the title of this PR says, this simple patch adds a missing `@implSpec` tag on `AddressLayout`, to bring it in sync with other layout classes.

-------------

Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/877/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=877&range=00
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/877.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/877/head:pull/877

PR: https://git.openjdk.org/panama-foreign/pull/877