[foreign-memaccess+abi] RFR: 8312615: Ensure jdk_foreign tests pass on linux-x86

[Jorn Vernee]

Ensure that the jdk_foreign tests pass on linux-x86.

This is useful to ensure that there are no hard to fix issues (that would require API changes) when trying to use the linker API on a 32-bit platform. This is a good indication that the FFM API is ready for finalization.

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Commit messages:
 - polish layout
 - use byteAlignement as indicator
 - linux-x86-debug fixes

Changes: https://git.openjdk.org/panama-foreign/pull/849/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=849&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312615
  Stats: 120 lines in 9 files changed: 61 ins; 30 del; 29 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/849.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/849/head:pull/849

PR: https://git.openjdk.org/panama-foreign/pull/849