RFR: 8318669: Target OS detection in 'test-prebuilt' makefile target is incorrect when running on MSYS2 [v2]

[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.

Frederic Thevenet has updated the pull request incrementally with one additional commit since the last revision:

  Use 'else ifeq' statement

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16343/files
  - new: https://git.openjdk.org/jdk/pull/16343/files/7a79d5f0..43aa74bb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16343&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16343&range=00-01

  Stats: 19 lines in 1 file changed: 6 ins; 8 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