[foreign-memaccess+abi] RFR: 8303017: Passing by-value structs whose size is not power of 2 doesn't work on all platforms [v2]

[Jorn Vernee]

> Fix passing of by-value structs whose size is not a power of 2.
> 
> The issue is that currently we try to do loads using a ValueLayout that is the size of the nearest power of two that can fit the struct (or part of it, if it is passed in multiple registers). For instance, for a struct of size 6, we try to load its value using `ValueLayout.OfLong`, which is size 8, and thus produce an out of bounds error. A similar issue applies to writes.
> 
> For the solution I've implemented in this patch, I've attached an explicit byte width to BufferLoads/Stores, which indicates the size of the value we want to load/store. The type that is produced by the binding is still the same. For example, loading a struct of size 6 is implemented as an `int` load and a `short` load, which are then combined into a `long`, instead of attempting to do a single `long` load (and failing). This allows us to avoid doing an out of bounds access.
> 
> I've added a new test that tests a bunch of structs with varying byte sizes being passed in registers and on the stack. Using a nested `char[]` to precisely tweak the byte size of each struct.

Jorn Vernee has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - beef up test
 - Merge branch 'foreign-memaccess+abi' into OOB
 - review comments
 - move pickChunkOffset to SharedUtils
 - eyeball BE support
 - forgot bufferStore for AArch64
 - add check + fix aarch64
 - polish
 - polish test
 - chunked in specializer
 - ... and 2 more: https://git.openjdk.org/panama-foreign/compare/a7df48c8...84be4370

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Changes: https://git.openjdk.org/panama-foreign/pull/806/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=806&range=01
  Stats: 706 lines in 11 files changed: 652 ins; 0 del; 54 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/806.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/806/head:pull/806

PR: https://git.openjdk.org/panama-foreign/pull/806