Patches to enable emulated-tls for older darwin systems in clang.
1) tell clang that TLS is supported on all systems
2) tell clang to pass -femulated-tls by default on systems older than 10.7
3) tell llvm to use cxa_thread_atexit instead of tlv_atexit on systems older than 10.7

requires linking against a std c++ lib that supports TLS and contains cxa_thread_atexit
(libgcc newer than 4.8, or libc++abi with cxa_thread_atexit added).

requires emutls.c objects available at link time to supply supporting routines for emulated-tls.
emutls is available in libgcc, and should be available in libc++abi for supporting emulated-tls

ideally : llvm should call cxa_thread_atexit rather than tlv_atexit based on 
          the -femulated-tls flag rather than based on the system version

these patches were originally formatted against clang-6.0
they can be applied to all clang builds, and will only affect systems 10.4 to 10.6 at present
note that because the patch to ItaniumCXXABI.cpp is dependent on the system version rather than
the -femultated-tls flag, it is not presently possible to force newer systems to use the 
emulated-tls code path


diff --git a/tools/clang/lib/Basic/Targets/OSTargets.h b/tools/clang/lib/Basic/Targets/OSTargets.h
index 5af63615..54c7ca66 100644
--- a/tools/clang/lib/Basic/Targets/OSTargets.h
+++ b/tools/clang/lib/Basic/Targets/OSTargets.h
@@ -93,7 +93,7 @@ public:
     this->TLSSupported = false;
 
     if (Triple.isMacOSX())
-      this->TLSSupported = !Triple.isMacOSXVersionLT(10, 7);
+      this->TLSSupported = !Triple.isMacOSXVersionLT(10, 4);
     else if (Triple.isiOS()) {
       // 64-bit iOS supported it from 8 onwards, 32-bit from 9 onwards.
       if (Triple.getArch() == llvm::Triple::x86_64 ||
diff --git a/tools/clang/lib/CodeGen/ItaniumCXXABI.cpp b/tools/clang/lib/CodeGen/ItaniumCXXABI.cpp
index a3c2766d..d73801df 100644
--- a/tools/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/tools/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -2186,7 +2186,8 @@ static void emitGlobalDtorWithCXAAtExit(CodeGenFunction &CGF,
   const char *Name = "__cxa_atexit";
   if (TLS) {
     const llvm::Triple &T = CGF.getTarget().getTriple();
-    Name = T.isOSDarwin() ?  "_tlv_atexit" : "__cxa_thread_atexit";
+    // Darwin 10.7+ has _tlv_atexit
+    Name = (T.isOSDarwin() && !T.isMacOSXVersionLT(10, 7)) ?  "_tlv_atexit" : "__cxa_thread_atexit";
   }
 
   // We're assuming that the destructor function is something we can
diff --git a/tools/clang/lib/Driver/ToolChains/Clang.cpp b/tools/clang/lib/Driver/ToolChains/Clang.cpp
index 7ee19085..7b357026 100644
--- a/tools/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/tools/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3859,9 +3859,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.AddLastArg(CmdArgs, options::OPT_fheinous_gnu_extensions);
   Args.AddLastArg(CmdArgs, options::OPT_fno_operator_names);
   // Emulated TLS is enabled by default on Android and OpenBSD, and can be enabled
-  // manually with -femulated-tls.
+  // manually with -femulated-tls. Also default on Darwin < 10.7
   bool EmulatedTLSDefault = Triple.isAndroid() || Triple.isOSOpenBSD() ||
-                            Triple.isWindowsCygwinEnvironment();
+     Triple.isMacOSXVersionLT(10, 7) || Triple.isWindowsCygwinEnvironment();
   if (Args.hasFlag(options::OPT_femulated_tls, options::OPT_fno_emulated_tls,
                    EmulatedTLSDefault))
     CmdArgs.push_back("-femulated-tls");