sim: Add relative break scheduling

Add schedRelBreak() function, executable within a debugger, that sets a
breakpoint by relative rather than absolute tick.

src/sim/debug.cc

@@ -89,6 +89,12 @@ schedBreak(Tick when)
     warn("need to stop all queues");
 }
 
+void
+schedRelBreak(Tick delta)
+{
+    schedBreak(curTick() + delta);
+}
+
 void
 breakAtKernelFunction(const char* funcName)
 {

src/sim/debug.hh

@@ -44,6 +44,13 @@
  */
 void schedBreak(Tick when);
 
+/**
+ * Cause the simulator to execute a breakpoint
+ * relative to the current tick.
+ * @param delta the number of ticks to execute until breaking
+ */
+void schedRelBreak(Tick delta);
+
 /**
  * Cause the simulator to execute a breakpoint when
  * the given kernel function is reached