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

```
----------------- 4741 -----------------
"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$1.run() bci:8 line:138 (Compiled frame)
* java.util.concurrent.ThreadPoolExecutor.runWorker(java.util.concurrent.ThreadPoolExecutor$Worker) bci:92 line:1128 (Compiled frame)
* java.util.concurrent.ThreadPoolExecutor$Worker.run() bci:5 line:628 (Compiled frame)
* java.lang.Thread.run() bci:11 line:834 (Compiled frame)
0x00007f95a17ce7e9 _ZN9JavaCalls11call_helperEP9JavaValueRK12methodHandleP17JavaCallArgumentsP6Thread + 0x3b9
0x00007f95a17cc79d _ZN9JavaCalls12call_virtualEP9JavaValue6HandleP5KlassP6SymbolS6_P6Thread + 0x1ed
0x00007f95a187685c _ZL12thread_entryP10JavaThreadP6Thread + 0x6c
0x00007f95a1d0cb0d _ZN10JavaThread17thread_main_innerEv + 0x21d
0x00007f95a1d0ceb7 _ZN10JavaThread3runEv + 0x377
0x00007f95a1b516a0 _ZL19thread_native_entryP6Thread + 0xf0
```

0x7f958c601d79 is native method. jstack mixed mode should handle it.