Support for stack scanning of roots.

Change-Id: Icbb02959725735faaa230e0fd00e8039b2f9c1b2
diff --git a/src/context.h b/src/context.h
index 05cd43b..b4a8b65 100644
--- a/src/context.h
+++ b/src/context.h
@@ -27,6 +27,9 @@
   // Set the program counter value
   virtual void SetPC(uintptr_t new_pc) = 0;
 
+  // Read the given GPR
+  virtual uintptr_t GetGPR(uint32_t reg) = 0;
+
   // Switch execution of the executing context to this context
   virtual void DoLongJump() = 0;
 };