RFR: 8318669: Target OS detection in 'test-prebuilt' makefile target is incorrect when running on MSYS2

[Frederic Thevenet]

This PR addresses OS detection not working properly when running test-prebuilt on MSYS2, which causes some Windows specific tests to misbehave or fail in Github Actions.

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Commit messages:
 - 8318669: Target OS detection in 'test-prebuilt' makefile target is incorrect when running on MSYS2

Changes: https://git.openjdk.org/jdk/pull/16343/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16343&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318669
  Stats: 18 lines in 1 file changed: 11 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/16343.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16343/head:pull/16343

PR: https://git.openjdk.org/jdk/pull/16343