Support for SetVReg and add SetGPR.

These changes are useful for debugging and are required for being able
to change all GC roots.

MIPS is untested.

Change-Id: I2ba055de64264098bffe869a4fb192d0975f1c8f
diff --git a/src/thread.cc b/src/thread.cc
index 7490d2a..5745c9b 100644
--- a/src/thread.cc
+++ b/src/thread.cc
@@ -1793,6 +1793,7 @@
     result = Context::Create();
   } else {
     long_jump_context_ = NULL;  // Avoid context being shared.
+    result->Reset();
   }
   return result;
 }