[foreign-abi] RFR: 8247993: AArch64: add support for VaList

[Nick Gasson]

Based on the SysV x64 implementation as it's somewhat similar to AArch64.

I added some extra tests to cover some cases on AArch64 that weren't hit
by the existing tests. The new testHugeStructByValue fails on Linux
x86_64 because the HugePoint struct is too large to be passed in
registers in the SysV ABI. Made a minimal fix to pass it on the stack
instead.

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Commit messages:
 - 8247993: AArch64: add support for VaList

Changes: https://git.openjdk.java.net/panama-foreign/pull/224/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/224/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247993
  Stats: 917 lines in 9 files changed: 817 ins; 86 del; 14 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/224.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/224/head:pull/224

PR: https://git.openjdk.java.net/panama-foreign/pull/224