Plumb use_system_xcode through native client.

src/trusted/service_runtime/BUILD.gn

Index: src/trusted/service_runtime/BUILD.gn
diff --git a/src/trusted/service_runtime/BUILD.gn b/src/trusted/service_runtime/BUILD.gn
index 268ca8940de3b5f7645b43f2a831b2328912ae1c..dc0cf715e37a1bb46d6a238e0e0ad6c17b90be55 100644
--- a/src/trusted/service_runtime/BUILD.gn
+++ b/src/trusted/service_runtime/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/toolchain/toolchain.gni")
 import("//native_client/src/trusted/asm.gni")
 
 source_set("sel_main") {
@@ -326,6 +327,9 @@ if (current_os == "mac") {
     ]
     args = rebase_path(sources, root_build_dir) +
            rebase_path(outputs, root_build_dir)
+    if (!use_system_xcode) {
+      args += [ hermetic_xcode_path ]
+    }
   }
 }