[foreign-memaccess+abi] RFR: Remove PlatformLayout [v4]

[Per Minborg]

> This PR suggests removing the class `PlatformLayout`.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Fix failing test

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/794/files
  - new: https://git.openjdk.org/panama-foreign/pull/794/files/99964974..695e07d6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=794&range=03
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=794&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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