Forward -u to the linker on gnutools toolchains

Summary:
The BSDs and Darwin all forward the whole 'u' group, but gcc only
forwards -u so far as I can tell.  I only forward -u, since that's a
minimal change, and many people object to magically recognizing and
forwarding linker arguments.

Reviewers: chandlerc, joerg

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4304

llvm-svn: 211756
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index f652fad..1f89d39 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -7158,6 +7158,7 @@
   }
 
   Args.AddAllArgs(CmdArgs, options::OPT_L);
+  Args.AddAllArgs(CmdArgs, options::OPT_u);
 
   const ToolChain::path_list Paths = ToolChain.getFilePaths();