Give compiler-rt a chance to compile with mingw-w64.
Patch by C. Bergström!

llvm-svn: 186463
diff --git a/compiler-rt/lib/eprintf.c b/compiler-rt/lib/eprintf.c
index b07d624..3626dbf 100644
--- a/compiler-rt/lib/eprintf.c
+++ b/compiler-rt/lib/eprintf.c
@@ -22,7 +22,9 @@
  * It should never be exported from a dylib, so it is marked
  * visibility hidden.
  */
+#ifndef _WIN32
 __attribute__((visibility("hidden")))
+#endif
 void __eprintf(const char* format, const char* assertion_expression,
 				const char* line, const char* file)
 {