Update implementation of atomics with latest Chromium version but use compiler builtin atomics

src/base/atomicops.h

Index: src/base/atomicops.h
diff --git a/src/base/atomicops.h b/src/base/atomicops.h
index 973e96b9ea5aad16b79d06474a2551ee3f289418..ea2af708ff4ed7d36fff4b7eec1628b620077c08 100644
--- a/src/base/atomicops.h
+++ b/src/base/atomicops.h
@@ -26,9 +26,17 @@
 #define V8_BASE_ATOMICOPS_H_
 
 #include <stdint.h>
+
+// Small C++ header which defines implementation specific macros used to
+// identify the STL implementation.
+// - libc++: captures __config for _LIBCPP_VERSION
+// - libstdc++: captures bits/c++config.h for __GLIBCXX__
+#include <cstddef>
+
+#include "src/base/base-export.h"
 #include "src/base/build_config.h"
 
-#if defined(_WIN32) && defined(V8_HOST_ARCH_64_BIT)
+#if defined(V8_OS_WIN) && defined(V8_HOST_ARCH_64_BIT)
 // windows.h #defines this (only on x64). This causes problems because the
 // public API also uses MemoryBarrier at the public name for this fence. So, on
 // X64, undef it, and call its documented
@@ -134,34 +142,17 @@ Atomic64 Release_Load(volatile const Atomic64* ptr);
 }  // namespace base
 }  // namespace v8
 
-// Include our platform specific implementation.
-#if defined(THREAD_SANITIZER)
-#include "src/base/atomicops_internals_tsan.h"
-#elif defined(_MSC_VER) && (V8_HOST_ARCH_IA32 || V8_HOST_ARCH_X64)
+#if defined(V8_OS_WIN)
+// TODO(jfb): The MSVC header includes windows.h, which other files end up
+//            relying on. Fix this as part of crbug.com/559247.
 #include "src/base/atomicops_internals_x86_msvc.h"
-#elif defined(__APPLE__)
-#include "src/base/atomicops_internals_mac.h"
-#elif defined(__GNUC__) && V8_HOST_ARCH_ARM64
-#include "src/base/atomicops_internals_arm64_gcc.h"
-#elif defined(__GNUC__) && V8_HOST_ARCH_ARM
-#include "src/base/atomicops_internals_arm_gcc.h"
-#elif defined(__GNUC__) && V8_HOST_ARCH_PPC
-#include "src/base/atomicops_internals_ppc_gcc.h"
-#elif defined(__GNUC__) && (V8_HOST_ARCH_IA32 || V8_HOST_ARCH_X64)
-#include "src/base/atomicops_internals_x86_gcc.h"
-#elif defined(__GNUC__) && V8_HOST_ARCH_MIPS
-#include "src/base/atomicops_internals_mips_gcc.h"
-#elif defined(__GNUC__) && V8_HOST_ARCH_MIPS64
-#include "src/base/atomicops_internals_mips64_gcc.h"
-#elif defined(__GNUC__) && V8_HOST_ARCH_S390
-#include "src/base/atomicops_internals_s390_gcc.h"
 #else
-#error "Atomic operations are not supported on your platform"
+#include "src/base/atomicops_internals_portable.h"
 #endif
 
 // On some platforms we need additional declarations to make
 // AtomicWord compatible with our other Atomic* types.
-#if defined(__APPLE__) || defined(__OpenBSD__) || defined(V8_OS_AIX)
+#if defined(V8_OS_MACOSX) || defined(V8_OS_OPENBSD) || defined(V8_OS_AIX)
 #include "src/base/atomicops_internals_atomicword_compat.h"
 #endif