[hurd] Add --build-id option when enabled

[kristina]

LGTM, at least following the same reasoning as in rC271692 (http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20160530/160984.html). I presume you want to be able to allow the default to be baked into the driver.

clang/lib/Driver/ToolChains/Hurd.cpp

Hurd::Hurd(const Driver &D, const llvm::Triple &Triple,
            const ArgList &Args)
     : Generic_ELF(D, Triple, Args) {
   std::string SysRoot = computeSysRoot();
   path_list &Paths = getFilePaths();
 
   const std::string OSLibDir = getOSLibDir(Triple, Args);
   const std::string MultiarchTriple = getMultiarchTriple(D, Triple, SysRoot);
 
+#ifdef ENABLE_LINKER_BUILD_ID
+  ExtraOpts.push_back("--build-id");
+#endif
+
   // If we are currently running Clang inside of the requested system root, add
   // its parent library paths to those searched.
   // FIXME: It's not clear whether we should use the driver's installed
   // directory ('Dir' below) or the ResourceDir.
   if (StringRef(D.Dir).startswith(SysRoot)) {
     addPathIfExists(D, D.Dir + "/../lib/" + MultiarchTriple, Paths);
     addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths);
   }