offwaketime does not show wakers of Java threads

When I profile a Java workload with `offwaketime`, wakers of Java threads become empty.

Component | Version
------------ | -------------
BCC tools | 0.10.0-1 (Upstream Ubuntu packages from repo.iovisor.org)
OS | Ubuntu 18.04.2 (Linux 4.15.0-43-generic)
Java | openjdk 12.0.1 2019-04-16
OpenJDK Runtime Environment AdoptOpenJDK (build 12.0.1+12)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 12.0.1+12, mixed mode, sharing)
perf-map-agent | Latest version (d9843a0)
https://github.com/jvm-profiling-tools/perf-map-agent

Below is the example pair of target and waker. The target contains frames of JIT-compiled code, but the waker is empty. Even frames in kernel are not included in the waker.

```
waker: 10904
-- --
finish_task_switch
schedule
exit_to_usermode_loop
prepare_exit_to_usermode
swapgs_restore_regs_and_return_to_usermode
ObjectSynchronizer::inflate(Thread*, oopDesc*, ObjectSynchronizer::InflateCause)
SharedRuntime::complete_monitor_locking_C(oopDesc*, BasicLock*, JavaThread*)
_complete_monitor_locking_Java
Lcom/ibm/websphere/jaxrs/server/IBMRestServlet;::doPost
Lcom/ibm/ws/webcontainer/servlet/ServletWrapper;::service
Lcom/ibm/ws/webcontainer/servlet/ServletWrapper;::handleRequest
Lcom/ibm/ws/webcontainer/filter/WebAppFilterChain;::invokeTarget
Lcom/ibm/ws/webcontainer/filter/WebAppFilterChain;::doFilter
Lcom/ibm/ws/webcontainer/filter/FilterInstanceWrapper;::doFilter
Lcom/ibm/ws/webcontainer/filter/WebAppFilterChain;::doFilter
Lcom/ibm/ws/webcontainer/filter/WebAppFilterManager;::doFilter
Lcom/ibm/ws/webcontainer/filter/WebAppFilterManager;::invokeFilters
Lcom/ibm/ws/webcontainer/servlet/CacheServletWrapper;::handleRequest
Lcom/ibm/ws/webcontainer/WebContainer;::handleRequest
Lcom/ibm/ws/webcontainer/osgi/DynamicVirtualHost$2;::run
Lcom/ibm/ws/http/dispatcher/internal/channel/HttpDispatcherLink;::ready
Lcom/ibm/ws/http/channel/internal/inbound/HttpInboundLink;::handleDiscrimination
Lcom/ibm/ws/http/channel/internal/inbound/HttpICLReadCallback;::complete
Lcom/ibm/ws/tcpchannel/internal/WorkQueueManager;::attemptIO
Lcom/ibm/ws/tcpchannel/internal/WorkQueueManager;::workerRun
Ljava/util/concurrent/ThreadPoolExecutor;::runWorker
Ljava/util/concurrent/ThreadPoolExecutor$Worker;::run
Ljava/lang/Thread;::run
call_stub
JavaCalls::call_helper(JavaValue*, methodHandle const&, JavaCallArguments*, Thread*)
JavaCalls::call_virtual(JavaValue*, Handle, Klass*, Symbol*, Symbol*, Thread*)
thread_entry(JavaThread*, Thread*)
JavaThread::run()
Thread::call_run()
thread_native_entry(Thread*)
start_thread
target: Default Executo 0
20
```

Below is another pair of waker and target in the same profile result. In this case, both waker and target frames are recorded, but the waker is not a Java thread. It looks like target only contains kernel frames.

```
waker: VM Thread 11644
start_thread
thread_native_entry(Thread*)
Thread::call_run()
VMThread::run()
VMThread::loop()
SafepointSynchronize::begin()
SafepointSynchronize::do_cleanup_tasks()
WorkGang::run_task(AbstractGangTask*)
SemaphoreGangTaskDispatcher::coordinator_execute_on_workers(AbstractGangTask*, unsigned int)
entry_SYSCALL_64_after_hwframe
do_syscall_64
sys_futex
do_futex
futex_wake
-- --
finish_task_switch
schedule
futex_wait_queue_me
futex_wait
do_futex
sys_futex
do_syscall_64
entry_SYSCALL_64_after_hwframe
do_futex_wait.constprop.1
target: GC Thread#6 11619
2618955
```

I am not sure this issue is specific to Java or not. Could anyone please advise how to diagnose this issue?

Start with the offwaketime entry point and the perf-map-agent setup described in the report. Reproduce the Java profile and compare stack collection for Java wakers, kernel frames, and the VM Thread example; done means the cause is identified and waker stacks are recorded consistently.

Written by the indexing model from the issue text.