[foreign-jextract] RFR: Fix some jextract test failures on Windows

[Jorn Vernee]

Hi,

This PR fixes some jextract test failure I was seeing locally, caused by use of `long` which has a different
size/carrier type on Windows.

MSVC also doesn't allow redefining `size_t` in a typedef, so I've changed the name of that.

Thanks,
Jorn

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Commit messages:
 - Fix some jextract test failures

Changes: https://git.openjdk.java.net/panama-foreign/pull/150/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/150/webrev.00
  Stats: 7 lines in 4 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/150.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/150/head:pull/150

PR: https://git.openjdk.java.net/panama-foreign/pull/150