Atomic: support clang 3.4+

Source/Core/Common/Atomic_GCC.h

@@ -45,7 +45,8 @@ inline void AtomicOr(volatile u32& target, u32 value) {
 	__sync_or_and_fetch(&target, value);
 }
 
-#ifdef __clang__
+// Support clang versions older than 3.4.
+#if __clang__ && !__has_feature(cxx_atomic)
 template <typename T>
 _Atomic(T)* ToC11Atomic(volatile T* loc)
 {

Source/Core/Common/Common.h

@@ -11,6 +11,10 @@
 #include <cstdlib>
 #include <cstring>
 
+#ifndef __has_feature
+#define __has_feature(x) 0
+#endif
+
 // SVN version number
 extern const char *scm_desc_str;
 extern const char *scm_branch_str;