[jdk21u] RFR: 8318669: Target OS detection in 'test-prebuilt' makefile target is incorrect when running on MSYS2

Sat Oct 28 10:26:55 UTC 2023

Backport of  [8318669: Target OS detection in 'test-prebuilt' makefile target is incorrect when running on MSYS2](https://bugs.openjdk.org/browse/JDK-8318669)

This is low risk as this only affects testing on Windows with MSYS2, when run using the "run-prebuilt" makefile target.
Without this patch, however, Windows-specific patches may fail or otherwise misbehave when run under such conditions (which is notably the case in Github actions).

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Commit messages:
 - Backport 202c0137b86cd7bcbe0c1eddf2657f45698ab667

Changes: https://git.openjdk.org/jdk21u/pull/299/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=299&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318669
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/299.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/299/head:pull/299

PR: https://git.openjdk.org/jdk21u/pull/299