8202290: AtomicAppend tests fail on macOS 10.13 ("High Sierra")

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8202290
http://cr.openjdk.java.net/~bpb/8202290/webrev.00/

Now that the macOS test machines which had been at version 10.13.x where x < 4 have been upgraded to x == 4, the patch [1] can be mostly reverted (leaving the additions to Platform in place).

Thanks,

Brian

[1] http://hg.openjdk.java.net/jdk/jdk/rev/3aaaa5370999