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

src/arm/assembler-thumb2-inl.h

Index: src/arm/assembler-thumb2-inl.h
===================================================================
--- src/arm/assembler-thumb2-inl.h	(revision 3473)
+++ src/arm/assembler-thumb2-inl.h	(working copy)
@@ -222,12 +222,20 @@
 
 
 void Assembler::emit(Instr x) {
+  forceArmMode();
   CheckBuffer();
   *reinterpret_cast<Instr*>(pc_) = x;
   pc_ += kInstrSize;
 }
 
+void Assembler::emitThumb(ThumbInstr x) {
+  forceThumbMode();
+  CheckBuffer();
+  *reinterpret_cast<ThumbInstr*>(pc_) = x;
+  pc_ += kThumbSize;
+}
 
+
 Address Assembler::target_address_address_at(Address pc) {
   Instr instr = Memory::int32_at(pc);
   // Verify that the instruction at pc is a ldr<cond> <Rd>, [pc +/- offset_12].