[Atomics] code stubs for atomic operations

src/arm/assembler-arm.cc

Index: src/arm/assembler-arm.cc
diff --git a/src/arm/assembler-arm.cc b/src/arm/assembler-arm.cc
index be85b96419798dade68820d4cae4ed93be1d7d48..625f35892bd14b0c6a11d96973f3d09bd560f022 100644
--- a/src/arm/assembler-arm.cc
+++ b/src/arm/assembler-arm.cc
@@ -2135,6 +2135,21 @@ void Assembler::svc(uint32_t imm24, Condition cond) {
 }
 
 
+void Assembler::dmb(BarrierOption option) {
+  emit(kSpecialCondition | 0x57ff*B12 | 5*B4 | option);
+}
+
+
+void Assembler::dsb(BarrierOption option) {
+  emit(kSpecialCondition | 0x57ff*B12 | 4*B4 | option);
+}
+
+
+void Assembler::isb(BarrierOption option) {
+  emit(kSpecialCondition | 0x57ff*B12 | 6*B4 | option);
+}
+
+
 // Coprocessor instructions.
 void Assembler::cdp(Coprocessor coproc,
                     int opcode_1,