lib/Basic/Targets.cpp: Set user_label_prefix on Win64 both mingw and msvc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123689 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 4faeac6..0552544 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -1579,6 +1579,7 @@
     SizeType = UnsignedLongLong;
     PtrDiffType = SignedLongLong;
     IntPtrType = SignedLongLong;
+    this->UserLabelPrefix = "";
   }
   virtual void getTargetDefines(const LangOptions &Opts,
                                 MacroBuilder &Builder) const {