Support for stack scanning of roots.

Change-Id: Icbb02959725735faaa230e0fd00e8039b2f9c1b2
diff --git a/src/context_arm.h b/src/context_arm.h
index 31a1a2a..07a4c0b 100644
--- a/src/context_arm.h
+++ b/src/context_arm.h
@@ -24,6 +24,12 @@
     gprs_[PC] = new_pc;
   }
 
+  virtual uintptr_t GetGPR(uint32_t reg) {
+    CHECK_GE(reg, 0u);
+    CHECK_LT(reg, 16u);
+    return gprs_[reg];
+  }
+
   virtual void DoLongJump();
 
  private: