[Cygwin] Use -femulated-tls by default since r257718 introduced the new pass.

FIXME: Add more targets to use emutls into clang/test/Driver/emulated-tls.cpp.
FIXME: Add cygwin tests into llvm/test/CodeGen/X86. Working in progress.
llvm-svn: 257984
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 90c2572..080fef8 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -4597,7 +4597,7 @@
   Args.AddLastArg(CmdArgs, options::OPT_fno_operator_names);
   // Emulated TLS is enabled by default on Android, and can be enabled manually
   // with -femulated-tls.
-  bool EmulatedTLSDefault = Triple.isAndroid();
+  bool EmulatedTLSDefault = Triple.isAndroid() || Triple.isWindowsCygwinEnvironment();
   if (Args.hasFlag(options::OPT_femulated_tls, options::OPT_fno_emulated_tls,
                    EmulatedTLSDefault))
     CmdArgs.push_back("-femulated-tls");