[foreign-memaccess+abi] RFR: Modernize code

[Per Minborg]

This PR proposes to modernize some parts of the code with constructs from newer Java versions.

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Commit messages:
 - Modernize code

Changes: https://git.openjdk.org/panama-foreign/pull/894/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=894&range=00
  Stats: 150 lines in 6 files changed: 21 ins; 26 del; 103 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/894.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/894/head:pull/894

PR: https://git.openjdk.org/panama-foreign/pull/894