[foreign-memaccess+abi] RFR: 8299911: Refactor struct handling in AArch64 CallArranger

[Jorn Vernee]

Refactor struct handling in AArch64 CallArranger.

Instead of having 6 very similar looking while loops, I've consolidated the storage allocation code for structs into a single helper method (`StorageCalculator::structStorages`). This should make it much easier to see which logic is being used for a particular case.

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Commit messages:
 - Add test comments
 - Fix2
 - Fix
 - fix comment
 - polish 2
 - polish
 - WIP
 - add test

Changes: https://git.openjdk.org/panama-foreign/pull/765/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=765&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299911
  Stats: 432 lines in 3 files changed: 181 ins; 160 del; 91 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/765.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/765/head:pull/765

PR: https://git.openjdk.org/panama-foreign/pull/765