core/sal: interface for backend-specific notion of time

This adds an interface for a backend-specific notion of time, e.g. CPU
cycles since simulator start, and a concrete implementation for the
Bochs backend.  This is needed to record CPU idle times (e.g., HLT
instruction), and for target backends capable of more timing-accurate
execution.

This change also modifies the tracing plugin to add the time to all
trace events.

Change-Id: I93ac1d54c07f32b0b8f84f333417741d8e9c8288

src/core/sal/bochs/BochsController.hpp

@@ -150,6 +150,8 @@ public:
 	 * @see The uses SimulatorController::getCPU().
 	 */
 	ConcreteCPU& detectCPU(BX_CPU_C* pCPU) const;
+	virtual simtime_t getTimerTicks() { return bx_pc_system.time_ticks(); }
+	virtual simtime_t getTimerTicksPerSecond() { return bx_pc_system.time_ticks() / bx_pc_system.time_usec(); /* imprecise hack */ }
 };
 
 } // end-of-namespace: fail