[foreign-jextract] RFR: Fix some tests failing jextract on Windows

[Jorn Vernee]

Hi,

This PR fixes some tests that were failing on Windows;

1. The new test for `long double` layouts gives a false positive on Windows for the `double` layout, since both are 64
bits. I've added another check to see that the `long double` layout is actually 128 bits. (Since it's 64 bits on
Windows, it would be allowed there, but it behaves the same as `double` so that should be fine).

2. A test using vsprintf was failing. It was depending on the default library to load the standard c library function,
but this doesn't work reliably on Windows. I've put a wrapper around the vsprintf function, and called that instead.
When building this wrapper the correct vsprintf is picked up.

Thanks,
Jorn

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Commit messages:
 - Fix some tests failing on Windows

Changes: https://git.openjdk.java.net/panama-foreign/pull/293/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/293/webrev.00
  Stats: 36 lines in 4 files changed: 32 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/293.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/293/head:pull/293

PR: https://git.openjdk.java.net/panama-foreign/pull/293