As a first step towards implementing thumb2, add code for...

src/arm/assembler-thumb2.h

Index: src/arm/assembler-thumb2.h
===================================================================
--- src/arm/assembler-thumb2.h	(revision 3473)
+++ src/arm/assembler-thumb2.h	(working copy)
@@ -508,6 +508,7 @@
 
 
 typedef int32_t Instr;
+typedef int16_t ThumbInstr;
 
 
 extern const Instr kMovLrPc;
@@ -590,6 +591,7 @@
 
   // Size of an instruction.
   static const int kInstrSize = sizeof(Instr);
+  static const int kThumbSize = sizeof(ThumbInstr);
 
   // Distance between the instruction referring to the address of the call
   // target (ldr pc, [target addr in const pool]) and the return address
@@ -996,11 +998,17 @@
   int written_position_;
   int written_statement_position_;
 
+  bool thumb_mode_;
+
   // Code emission
   inline void CheckBuffer();
   void GrowBuffer();
   inline void emit(Instr x);
+  inline void emitThumb(ThumbInstr x);
 
+  void forceThumbMode();
+  void forceArmMode();
+
   // Instruction generation
   void addrmod1(Instr instr, Register rn, Register rd, const Operand& x);
   void addrmod2(Instr instr, Register rd, const MemOperand& x);