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/process.cpp

@@ -80,6 +80,14 @@ u64 Process::GetTotalPhysicalMemoryUsed() const {
     return vm_manager.GetCurrentHeapSize() + main_thread_stack_size + code_memory_size;
 }
 
+void Process::RegisterThread(const Thread* thread) {
+    thread_list.push_back(thread);
+}
+
+void Process::UnregisterThread(const Thread* thread) {
+    thread_list.remove(thread);
+}
+
 ResultCode Process::ClearSignalState() {
     if (status == ProcessStatus::Exited) {
         LOG_ERROR(Kernel, "called on a terminated process instance.");