Add Dummy implementations of atomics for Cobalt

This is part of Cobalt's upstream effort to be more in sync with V8.
Cobalt does not trigger the atomics functions in runtime-atomics.cc so
we use dummy implementations for now.

Bug: v8:10927
Change-Id: I2da75b7f9b22b41a1ff00af1ab4b43641e9a33bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2963801
Commit-Queue: John Xu <johnx@google.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75317}

src/base/atomicops.h

@@ -41,10 +41,6 @@
 
 #if defined(V8_OS_STARBOARD)
 #include "starboard/atomic.h"
-
-#if SB_API_VERSION < 10
-#error Your version of Starboard must support SbAtomic8 in order to use V8.
-#endif  // SB_API_VERSION < 10
 #endif  // V8_OS_STARBOARD
 
 namespace v8 {

src/runtime/runtime-atomics.cc

@@ -24,7 +24,44 @@ namespace internal {
 
 namespace {
 
-#if V8_CC_GNU
+#if defined(V8_OS_STARBOARD)
+
+template <typename T>
+inline T ExchangeSeqCst(T* p, T value) {
+  UNIMPLEMENTED();
+}
+
+template <typename T>
+inline T CompareExchangeSeqCst(T* p, T oldval, T newval) {
+  UNIMPLEMENTED();
+}
+
+template <typename T>
+inline T AddSeqCst(T* p, T value) {
+  UNIMPLEMENTED();
+}
+
+template <typename T>
+inline T SubSeqCst(T* p, T value) {
+  UNIMPLEMENTED();
+}
+
+template <typename T>
+inline T AndSeqCst(T* p, T value) {
+  UNIMPLEMENTED();
+}
+
+template <typename T>
+inline T OrSeqCst(T* p, T value) {
+  UNIMPLEMENTED();
+}
+
+template <typename T>
+inline T XorSeqCst(T* p, T value) {
+  UNIMPLEMENTED();
+}
+
+#elif V8_CC_GNU
 
 // GCC/Clang helpfully warn us that using 64-bit atomics on 32-bit platforms
 // can be slow. Good to know, but we don't have a choice.