[backport] SI-7497 Fix scala.util.Properties.isMac

It returned wrong values because it used the java.vendor
property instead of os.name, as specified in
developer.apple.com/library/mac/#technotes/tn2002/tn2110.
Merge pull request #2560 from soc/SI-7497-2.9

[backport] SI-7497 Fix scala.util.Properties.isMac