RFR: 8232592: <Unknown compiled code> is shown in jstack mixed mode

[Yasumasa Suenaga]

Hi all,

Please review this change.

   JBS: https://bugs.openjdk.java.net/browse/JDK-8232592
   webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8232592/webrev.00/

I run `jhsdb jstack --mixed` to JShellToolProvider process, then I saw <Unknown compiled code>
in the stack as below:

```
"process reaper" #13 daemon prio=10 tid=0x00007f959c328000 nid=0x1285 runnable [0x00007f9578025000]
    java.lang.Thread.State: RUNNABLE
    JavaThread state: _thread_in_native
0x00007f95a27c5596 __waitpid + 0x56
0x00007f958c601d79 <Unknown compiled code>
0x00007f9585c5b4ac * java.lang.ProcessHandleImpl$1.run() bci:8 line:138 (Compiled frame)
```

0x7f958c601d79 is native method. jstack mixed mode should handle it.
This patch passed all tests on submit repo (mach5-one-ysuenaga-JDK-8232592-20191018-0600-6011163).


Thanks,

Yasumasa