Basic: fix __USER_LABEL_PREFIX__ on Cygwin

Adjust the user label prefix for cygwin x86_64.

Resolves PR26744.

llvm-svn: 262030
diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp
index 46532ff..040deda 100644
--- a/clang/lib/Basic/Targets.cpp
+++ b/clang/lib/Basic/Targets.cpp
@@ -4226,6 +4226,7 @@
       : X86_64TargetInfo(Triple) {
     TLSSupported = false;
     WCharType = UnsignedShort;
+    UserLabelPrefix = "";
   }
   void getTargetDefines(const LangOptions &Opts,
                         MacroBuilder &Builder) const override {