Kernel: Tear down process address space during finalization

Process teardown is divided into two main stages: finalize and reap.

Finalization happens in the "Finalizer" kernel and runs with interrupts
enabled, allowing destructors to take locks, etc.

Reaping happens either in sys$waitid() or in the scheduler for orphans.

The more work we can do in finalization, the better, since it's fully
pre-emptible and reduces the amount of time the system runs without
interrupts enabled.

Kernel/Process.cpp

@@ -3016,6 +3016,8 @@ void Process::finalize()
         }
     }
 
+    m_regions.clear();
+
     m_dead = true;
 }