[arm] Implement barriers on ARMv6 using CP15.

src/arm/assembler-arm.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions
 // are met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 1004 matching lines @@
   void stm(BlockAddrMode am, Register base, RegList src, Condition cond = al);
 
   // Exception-generating instructions and debugging support
   void stop(const char* msg,
             Condition cond = al,
             int32_t code = kDefaultStopCode);
 
   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);
 
   // Coprocessor instructions
 
   void cdp(Coprocessor coproc, int opcode_1,
            CRegister crd, CRegister crn, CRegister crm,
            int opcode_2, Condition cond = al);
 
@@ skipping 649 matching lines @@
   explicit EnsureSpace(Assembler* assembler) {
     assembler->CheckBuffer();
   }
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_ARM_ASSEMBLER_ARM_H_