[foreign-memaccess+abi] RFR: Convert local classes to proper utility classes

[Per Minborg]

This PR proposes to convert local holder classes to proper utility classes. 

The utility of this PR can be discussed as there are valid arguments for keeping the code as it is and also to accept the PR.

Let me know your thoughts.

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Commit messages:
 - Convert local classes to proper utility classes

Changes: https://git.openjdk.org/panama-foreign/pull/904/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=904&range=00
  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/904.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/904/head:pull/904

PR: https://git.openjdk.org/panama-foreign/pull/904