update comment

Author: Un1q32

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -1645,9 +1645,11 @@ void DarwinClang::AddLinkRuntimeLibArgs(const ArgList &Args,
     CmdArgs.push_back("-lSystem");
 
   // Select the dynamic runtime library and the target specific static library.
-  // If we are compiling as iOS / simulator, don't attempt to link libgcc_s.1,
-  // it never went into the SDK.
-  // Linking against libgcc_s.1 isn't needed for iOS 5.0+ or macOS 10.6+
+  // Some old Darwin versions put builtins, libunwind, and some other stuff in
+  // libgcc_s.1.dylib. MacOS X 10.6 and iOS 5 moved those functions to
+  // libSystem, and made libgcc_s.1.dylib a stub. We never link libgcc_s when
+  // building for aarch64 or iOS simulator, since libgcc_s was made obsolete
+  // before either existed.
   if (getTriple().getArch() != llvm::Triple::aarch64 &&
       ((isTargetIOSBased() && isIPhoneOSVersionLT(5, 0) &&
         !isTargetIOSSimulator()) ||