Enable outline atomics on OpenBSD/arm64 by default.

ok patrick@, deraadt@

gnu/llvm/clang/lib/Driver/ToolChains/OpenBSD.h

@@ -79,6 +79,11 @@ public:
   void AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
                            llvm::opt::ArgStringList &CmdArgs) const override;
 
+  bool IsAArch64OutlineAtomicsDefault(
+      const llvm::opt::ArgList &Args) const override {
+    return true;
+  }
+
   std::string getCompilerRT(const llvm::opt::ArgList &Args, StringRef Component,
                             FileType Type = ToolChain::FT_Static) const override;