[foreign-memaccess+abi] RFR: Rework PlatformLayout

[Per Minborg]

This PR suggests slightly reworking the class `PlatformLayout`. This reduces the number of objects created and aggregates common functionality in a support method.

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Commit messages:
 - Rework PlatformLayout

Changes: https://git.openjdk.org/panama-foreign/pull/794/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=794&range=00
  Stats: 70 lines in 1 file changed: 7 ins; 23 del; 40 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/794.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/794/head:pull/794

PR: https://git.openjdk.org/panama-foreign/pull/794