[foreign-memaccess+abi] RFR: Rename max alignment constants for heap segments

[Maurizio Cimadamore]

The name of the max alignment constants in `HeapMemorySegmentImpl` is misleading: these constants use numeric values (e.g. `MAX_ALIGN_8`) but in reality the maximum supported alignment can vary depending on the platform (at least for segments wrapping `long[]` and `double[]` arrays).

This PR introduces a less confusing naming scheme, and a comment to call out the fact that some constants are platform-dependent.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/897/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=897&range=00
  Stats: 17 lines in 1 file changed: 5 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/897.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/897/head:pull/897

PR: https://git.openjdk.org/panama-foreign/pull/897