Forward -u to the linker on gnutools toolchains

[rnk]

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.

[echristo]

LGTM.

[rnk]

Closed by commit rL211756 (authored by @rnk).

lib/Driver/Tools.cpp

else
       crtbegin = isAndroid ? "crtbegin_dynamic.o" : "crtbegin.o";
     CmdArgs.push_back(Args.MakeArgString(ToolChain.GetFilePath(crtbegin)));
 
     // Add crtfastmath.o if available and fast math is enabled.
     ToolChain.AddFastMathRuntimeIfAvailable(Args, CmdArgs);
   }
 
   Args.AddAllArgs(CmdArgs, options::OPT_L);
+  Args.AddAllArgs(CmdArgs, options::OPT_u);
 
   const ToolChain::path_list Paths = ToolChain.getFilePaths();
 
   for (const auto &Path : Paths)
     CmdArgs.push_back(Args.MakeArgString(StringRef("-L") + Path));
 
   if (D.IsUsingLTO(Args))
     AddGoldPlugin(ToolChain, Args, CmdArgs);

test/Driver/linux-ld.c

 // CHECK-CRTFASTMATH: usr/lib/gcc/x86_64-unknown-linux/4.6.0{{/|\\\\}}crtfastmath.o
 // CHECK-NOCRTFASTMATH-NOT: crtfastmath.o
 
 // Check that we link in gcrt1.o when compiling with -pg
 // RUN: %clang -pg --target=x86_64-unknown-linux -### %s \
 // RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>& 1 \
 // RUN:   | FileCheck --check-prefix=CHECK-PG %s
 // CHECK-PG: gcrt1.o
+
+// GCC forwards -u to the linker.
+// RUN: %clang -u asdf --target=x86_64-unknown-linux -### %s \
+// RUN:        --sysroot=%S/Inputs/basic_linux_tree 2>& 1 \
+// RUN:   | FileCheck --check-prefix=CHECK-u %s
+// CHECK-u: "-u" "asdf"