Fix threads detection on GNU/Hurd

GNU/Hurd provides standard Posix threads

https://reviews.llvm.org/D54339

Index: llvm-toolchain-7-7/libcxx/include/__config
===================================================================
--- llvm-toolchain-7-7.orig/libcxx/include/__config
+++ llvm-toolchain-7-7/libcxx/include/__config
@@ -1110,6 +1110,7 @@ _LIBCPP_FUNC_VIS extern "C" void __sanit
       defined(__Fuchsia__) || \
       defined(__NetBSD__) || \
       defined(__linux__) || \
+      defined(__GNU__) || \
       defined(__APPLE__) || \
       defined(__CloudABI__) || \
       defined(__sun__) || \