[foreign-memaccess+abi] RFR: 8303888: Add AddressLayout.withoutTargetLayout

[Per Minborg]

This PR proposes to add a `AddressLayout.withoutTargetLayout()` method for symmetry reasons. This allows  AddressLayout to be converted back to raw addresses.

Discussion point: Should this method require native access?

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Commit messages:
 - Add AddressLayout.withoutTargetLayout

Changes: https://git.openjdk.org/panama-foreign/pull/815/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=815&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303888
  Stats: 16 lines in 3 files changed: 16 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/815.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/815/head:pull/815

PR: https://git.openjdk.org/panama-foreign/pull/815