[foreign-memaccess+abi] RFR: 8296973: saving errno on a value-returning function crashes the JVM

[Jorn Vernee]

There are 2 bugs:

- Frame allocation code is not in the right order. There's an optimization where the area used for spilling return values around slow path calls is shared with the shadow space and stack args area. This is possible since the latter are only used before the native call, and the former only after. But, the order of the code that allocates this space is incorrect, which means that the spill area for return values is shared with _all_ the data in the frame. This also recently includes the address of the captured state segment (for `errno`). In case there are no stack arguments and no shadow space (as on linux in most cases), this means in practice that a spill of the return value will overwrite the capture state address. (previously we had also the return buffer address, but use of a return buffer and spilling of return values are mutually exclusive, so it was never a problem).
- The `CaptureCallState` linker option adds an additional leading parameter (the captured state segment), which can mess up `SharedUtils::adaptDowncallForIMR`.

I've extended that CaptureCallState test to include the most interesting cases: simple scalar returns, and various struct returns that are: returned in a single register (no return buffer), returned in multiple registers (yes return buffer), or returned through ABI-level in-memory-return pointers (using `SharedUtils::adaptDowncallForIMR`).

I've also done a big sanity test where I adapted `TestDowncallScope` to also set and capture `errno` with each call. That seemed overkill to include in the repo though (?)

Thanks

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Commit messages:
 - Rejig frame allocation code

Changes: https://git.openjdk.org/panama-foreign/pull/753/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=753&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296973
  Stats: 198 lines in 9 files changed: 161 ins; 22 del; 15 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/753.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/753/head:pull/753

PR: https://git.openjdk.org/panama-foreign/pull/753