[foreign-abi] RFR: 8233007: Add upcall tests that test stack arguments on Windows

[Jorn Vernee]

Hi,

This PR adds a test for high-arity upcalls.

This actually uncovered a bug in the Windows CallArranger wrt passing structs by-value on the stack, which I've fixed.
I've also added the particular test case to the CallArranger test.

I've also fixed an unsigned comparison warning I got when building on WSL.

Thanks,
Jorn

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Commit messages:
 - Add a test for high-arity upcalls

Changes: https://git.openjdk.java.net/panama-foreign/pull/342/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=342&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8233007
  Stats: 298 lines in 7 files changed: 267 ins; 29 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/342.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/342/head:pull/342

PR: https://git.openjdk.java.net/panama-foreign/pull/342