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

ARM specific CONFIG_KUSER_HELPERS kernel feature for Linux can be disabled,
and in this case, we shouldn't crash. Use a __sync_synchronize() call
instead for Linux platforms.

BUG=chromium:599051
LOG=Y

Review URL: https://codereview.chromium.org/1840203004

Cr-Commit-Position: refs/heads/master@{#35170}

AUTHORS

@@ -71,6 +71,7 @@ Jianghua Yang <jianghua.yjh@alibaba-inc.com>
 Joel Stanley <joel@jms.id.au>
 Johan Bergström <johan@bergstroem.nu>
 Jonathan Liu <net147@gmail.com>
+Julien Brianceau <jbriance@cisco.com>
 JunHo Seo <sejunho@gmail.com>
 Kang-Hao (Kenny) Lu <kennyluck@csail.mit.edu>
 Karl Skomski <karl@skomski.com>

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
 }