[arm/Linux] Don't rely on KUSER_HELPERS feature

src/base/atomicops_internals_arm_gcc.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file is an internal atomic implementation, use atomicops.h instead.
 //
 // LinuxKernelCmpxchg and Barrier_AtomicIncrement are from Google Gears.
 
 #ifndef V8_BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_
 #define V8_BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_
@@ skipping 26 matching lines @@
 //   core devices, this is an empty function that exits immediately.
 //   On multi-core devices, it implements a full memory barrier.
 //
 // * This source could be compiled to ARMv5 machine code that runs on a
 //   multi-core ARMv6 or ARMv7 device. In this case, memory barriers
 //   are needed for correct execution. Always call the kernel helper, even
 //   when targeting ARMv5TE.
 //
 
 inline void MemoryBarrier() {
-#if defined(__linux__) || defined(__ANDROID__)
+#if defined(__ANDROID__)
   // Note: This is a function call, which is also an implicit compiler barrier.
   typedef void (*KernelMemoryBarrierFunc)();
   ((KernelMemoryBarrierFunc)0xffff0fa0)();
 #elif defined(__QNXNTO__)
   __cpu_membarrier();
 #else
-#error MemoryBarrier() is not implemented on this platform.
+  // Fallback to GCC built-in function
+  __sync_synchronize();
 #endif
 }
 
 // An ARM toolchain would only define one of these depending on which
 // variant of the target architecture is being used. This tests against
 // any known ARMv6 or ARMv7 variant, where it is possible to directly
 // use ldrex/strex instructions to implement fast atomic operations.
 #if defined(__ARM_ARCH_8A__) || \
     defined(__ARM_ARCH_7__) || defined(__ARM_ARCH_7A__) ||  \
     defined(__ARM_ARCH_7R__) || defined(__ARM_ARCH_7M__) || \
@@ skipping 229 matching lines @@
 inline void NoBarrier_Store(volatile Atomic8* ptr, Atomic8 value) {
   *ptr = value;
 }
 
 inline Atomic8 NoBarrier_Load(volatile const Atomic8* ptr) { return *ptr; }
 
 }  // namespace base
 }  // namespace v8
 
 #endif  // V8_BASE_ATOMICOPS_INTERNALS_ARM_GCC_H_