[arm] Implement barriers on ARMv6 using CP15.

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 0b9cd91733073118328eb43aaed05b528ccfc748..e77b49da5e6f8ab72eb9362a68fbcc235dc8372e 100644
--- a/src/arm/assembler-arm.h
+++ b/src/arm/assembler-arm.h
@@ -1022,7 +1022,8 @@ class Assembler : public AssemblerBase {
   void bkpt(uint32_t imm16);  // v5 and above
   void svc(uint32_t imm24, Condition cond = al);
 
-  // Synchronization instructions
+  // Synchronization instructions.
+  // On ARMv6, an equivalent CP15 operation will be used.
   void dmb(BarrierOption option);
   void dsb(BarrierOption option);
   void isb(BarrierOption option);