Various assembly fixes.

Change-Id: I42655af8dc3d7201d53c3393cdc585d9faa8cb82
diff --git a/src/assembler_arm.h b/src/assembler_arm.h
index 3f979c6..e1d43e4 100644
--- a/src/assembler_arm.h
+++ b/src/assembler_arm.h
@@ -346,6 +346,7 @@
   void b(Label* label, Condition cond = AL);
   void bl(Label* label, Condition cond = AL);
   void blx(Register rm, Condition cond = AL);
+  void bx(Register rm, Condition cond = AL);
 
   // Macros.
   // Add signed constant value to rd. May clobber IP.