RFR: Use compiler switch to filter out `__asm` statements on Windows

[Jorn Vernee]

A newly added test uses `__asm` statements which are not supported by MSVC. I failed to catch this during the review of that patch, and this result in a compilation failure on Windows in the most recent version of the code.

The test that needs this native code doesn't run on Windows any way, so I've removed all the Windows-isms from the native code, and instead just disabled compilation of the problematic code on Windows using a compiler switch.

Alternatively, we could modify the `CMakeLists.txt` file to disable building this particular library on Windows, if that's preferred.

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Commit messages:
 - Use compiler switch to filter out __asm statements

Changes: https://git.openjdk.org/jextract/pull/234/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=234&range=00
  Stats: 15 lines in 2 files changed: 2 ins; 4 del; 9 mod
  Patch: https://git.openjdk.org/jextract/pull/234.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/234/head:pull/234

PR: https://git.openjdk.org/jextract/pull/234