[foreign-memaccess+abi] RFR: 8275584: Incorrect stack spilling in CallArranger on MacOS/AArch64

[Nick Gasson]

This patch adds special handling for argument spilling on M1 Mac which does not follow the standard AArch64.  In the standard ABI arguments are first extended to the full 64-bit register width and then spilled but on macOS the arguments are spilled according to their original width and packed using theior natural.  @JornVernee  did most of the work to support this but there were a few issues remaining related to structspilling which I've fixed up in the last commit here.

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Commit messages:
 - 8275584: Incorrect stack spilling in CallArranger on MacOS/AArch64
 - index to offset
 - fix part 2
 - Attempt fix

Changes: https://git.openjdk.org/panama-foreign/pull/746/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=746&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8275584
  Stats: 306 lines in 5 files changed: 247 ins; 24 del; 35 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/746.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/746/head:pull/746

PR: https://git.openjdk.org/panama-foreign/pull/746