kernel/svc: Implement svcGetThreadList

Similarly like svcGetProcessList, this retrieves the list of threads
from the current process. In the kernel itself, a process instance
maintains a list of threads, which are used within this function.

Threads are registered to a process' thread list at thread
initialization, and unregistered from the list upon thread destruction
(if said thread has a non-null owning process).

We assert on the debug event case, as we currently don't implement
kernel debug objects.

src/core/hle/kernel/thread.cpp

@@ -62,6 +62,8 @@ void Thread::Stop() {
     }
     wait_objects.clear();
 
+    owner_process->UnregisterThread(this);
+
     // Mark the TLS slot in the thread's page as free.
     owner_process->FreeTLSSlot(tls_address);
 }
@@ -202,6 +204,8 @@ ResultVal<SharedPtr<Thread>> Thread::Create(KernelCore& kernel, std::string name
     thread->scheduler->AddThread(thread);
     thread->tls_address = thread->owner_process->MarkNextAvailableTLSSlotAsUsed(*thread);
 
+    thread->owner_process->RegisterThread(thread.get());
+
     // TODO(peachum): move to ScheduleThread() when scheduler is added so selected core is used
     // to initialize the context
     ResetThreadContext(thread->context, stack_top, entry_point, arg);