[foreign-abi] RFR: 8248420: Add a variant of VaList::make which takes a NativeScope

[Jorn Vernee]

Hi,

This patch adds overloads to several VaList related methods that might need to do allocations, namely, VaList::make,
VaList::vargAsSegment, and VaList::copy.

In order to share the existing code, I've added an `Allocator` helper interface that can wrap either a NativeScope, or
be used as a functional interface to bind to MemorySegment::allocateNative.

While testing this patch, I've also discovered a bug in the SysV implementation; The VaList::Builder was allocating a
MemorySegment for the VaList, then creating the SysVVaList object wrapping that segment, and then filling in some of
the contents of the segment. But, the SysVVaList constructor was also trying to read from the segment as well, and as a
result was reading uninitialized fields. This is a problem when creating a VaList in Java, and then trying to read from
it also in Java.

I've restructured the code a bit to fix this problem, moving the reading logic from the constructor to a static factory
called readFromSegment, so that it's clear that only a fully initialized segment should be passed. The builder still
calls the constructor directly, but it now requires all field values to be passed in explicitly.

There was a similar problem in the AArch64 implementation, which I've tried to fix. Unfortunately I was not able to
fully test this change since there was a failure elsewhere in the testing infrastructure, and I currently don't have
direct access to an aarch64 machine to be able to debug that. I did end up fixing some build failures on aarch64 in the
process though, which I will submit another PR for.

Thanks,
Jorn

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Commit messages:
 - Add NativeScope overloads for VaList methods that need to allocate

Changes: https://git.openjdk.java.net/panama-foreign/pull/237/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/237/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248420
  Stats: 295 lines in 9 files changed: 223 ins; 12 del; 60 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/237.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/237/head:pull/237

PR: https://git.openjdk.java.net/panama-foreign/pull/237