[foreign-memaccess+abi] RFR: Remove PlatformLayout [v5]

[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:

  Reuse existing C_POINTER

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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/695e07d6..1efba50b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=794&range=04
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=794&range=03-04

  Stats: 12 lines in 1 file changed: 2 ins; 6 del; 4 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