Bug 1758780 - Fallthrough to <atomic> for wasi when using clang >= 14…

…. r=nika

Clang 14 added a dummy backoff policy for atomic polling that doesn't
know about threads, which presumably WASI can use.
See https://reviews.llvm.org/D114109.

Differential Revision: https://phabricator.services.mozilla.com/D143173

mfbt/WasiAtomic.h

@@ -7,7 +7,12 @@
 #ifndef mozilla_WasiAtomic_h
 #define mozilla_WasiAtomic_h
 
-#include <cstdint>
+// Clang >= 14 supports <atomic> for wasm targets.
+#if _LIBCPP_VERSION >= 14000
+#  include <atomic>
+#else
+
+#  include <cstdint>
 
 // WASI doesn't support <atomic> and we use it as single-threaded for now.
 // This is a stub implementation of std atomics to build WASI port of SM.
@@ -189,4 +194,6 @@ using atomic_uint64_t = atomic<uint64_t>;
 
 }  // namespace std
 
+#endif
+
 #endif  // mozilla_WasiAtomic_h