RFR: 8257620: Do not use objc_msgSend_stret to get macOS version

[Anton Kozlov]

Please review a small change that replaces use of objc_msgSend_stret in macOS platform code with pure ObjC code. It's also a prerequisite for macOS/AArch64 support, where objc_msgSend_stret is not available.

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Commit messages:
 - Do not use objc_msgSend_stret to get macOS version

Changes: https://git.openjdk.java.net/jdk/pull/1569/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1569&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257620
  Stats: 9 lines in 1 file changed: 5 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1569.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1569/head:pull/1569

PR: https://git.openjdk.java.net/jdk/pull/1569