Use dmb.

We don't support ARM processors that don't have dmb anyway.

Change-Id: I41b15a1592febcc0b33d93c291431d00692a2928
diff --git a/compiler/utils/arm/assembler_arm.cc b/compiler/utils/arm/assembler_arm.cc
index f0d11d8..828dffa 100644
--- a/compiler/utils/arm/assembler_arm.cc
+++ b/compiler/utils/arm/assembler_arm.cc
@@ -1742,17 +1742,8 @@
 void ArmAssembler::MemoryBarrier(ManagedRegister mscratch) {
   CHECK_EQ(mscratch.AsArm().AsCoreRegister(), R12);
 #if ANDROID_SMP != 0
-#if defined(__ARM_HAVE_DMB)
   int32_t encoding = 0xf57ff05f;  // dmb
   Emit(encoding);
-#elif  defined(__ARM_HAVE_LDREX_STREX)
-  LoadImmediate(R12, 0);
-  int32_t encoding = 0xee07cfba;  // mcr p15, 0, r12, c7, c10, 5
-  Emit(encoding);
-#else
-  LoadImmediate(R12, 0xffff0fa0);  // kuser_memory_barrier
-  blx(R12);
-#endif
 #endif
 }