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

src/base/atomicops_internals_arm_gcc.h

Index: src/base/atomicops_internals_arm_gcc.h
diff --git a/src/base/atomicops_internals_arm_gcc.h b/src/base/atomicops_internals_arm_gcc.h
index 6c8b27ea24ec69512e0339ccc17f6dc366e0fd90..8d049e04b48b7ebf7e58f902617218eca1e33730 100644
--- a/src/base/atomicops_internals_arm_gcc.h
+++ b/src/base/atomicops_internals_arm_gcc.h
@@ -44,14 +44,15 @@ namespace base {
 //
 
 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
 }