Cache KFD Events used by user allocated InterruptSignals.

Change-Id: I7f102f880fea9c78febe28cd262f93ee77f03184

runtime/hsa-runtime/core/runtime/runtime.cpp

@@ -1221,6 +1221,8 @@ void Runtime::Unload() {
 
   SharedSignalPool.clear();
 
+  EventPool.clear();
+
   DestroyAgents();
 
   CloseTools();