JCLOUDS-658: use xattr only on supported systems

Mac OS X HFS+ does not support UserDefinedFileAttributeView:

https://bugs.openjdk.java.net/browse/JDK-8030048
I was wondering if there's a way to condense these two lines down into a `getViewIfAvailable` method or whatever, but I'm not sure the result is that much nicer. Just a style thought, in any case.