MFH: r405058, r405320

• Stub implementation of OperatingSystemImpl.
• Partially implement getThreadUserTime() using getrusage(2). Note we can

only get usage for the current thread. Return -1 if the requested function
is not supported, i.e., user time for other threads, rather than crash.

• Properly implement os::elapsedVTime() using getrusage(). Basically, it is

taken from Linux version.

• Temporarily revert r403748 to fix bootstrapping with earlier OpenJDK8.

PR: 205229, 205523, 205544, 205843
Approved by: ports-secteam (feld)