[Atomics] code stubs for atomic operations

src/arm/assembler-arm.h

Index: src/arm/assembler-arm.h
diff --git a/src/arm/assembler-arm.h b/src/arm/assembler-arm.h
index e0cfe8c36318d94914b81244ad34b3c52938f358..e0b56262c5f824c84b192890a5084303821f760a 100644
--- a/src/arm/assembler-arm.h
+++ b/src/arm/assembler-arm.h
@@ -986,6 +986,11 @@ class Assembler : public AssemblerBase {
   void bkpt(uint32_t imm16);  // v5 and above
   void svc(uint32_t imm24, Condition cond = al);
 
+  // Synchronization instructions
+  void dmb(BarrierOption option);
+  void dsb(BarrierOption option);
+  void isb(BarrierOption option);
+
   // Coprocessor instructions
 
   void cdp(Coprocessor coproc, int opcode_1,