Handle different format of OperatingSystemMXBean.getSystemLoadAverage() output on macosx

OperatingSystemMXBean.getSystemLoadAverage() uses /usr/bin/uptime on Unix, Linux, and Mac OS X. The Mac OS X output text is slightly different, but the regression test in test/java/lang/management/OperatingSystemMXBean/GetSystemLoadAverage.java only handles the output format for non-Mac OS output. Add handling of the output on Mac OS X.