[foreign-memaccess+abi] RFR: Address failures in jdk.incubator.foreign benchmarks

[Julia Boes]

This change addresses some failures in the jdk.incubator.foreign benchmarks, mainly caused by missing casts and missing native access.

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Commit messages:
 - initial change

Changes: https://git.openjdk.java.net/panama-foreign/pull/602/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=602&range=00
  Stats: 39 lines in 9 files changed: 1 ins; 10 del; 28 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/602.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/602/head:pull/602

PR: https://git.openjdk.java.net/panama-foreign/pull/602