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

[Maurizio Cimadamore]

On Thu, 16 Feb 2023 14:56:34 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> 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

Marked as reviewed by mcimadamore (Committer).

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