gn: Support build directories outside the source tree.

tools/gn/function_set_default_toolchain.cc

Index: tools/gn/function_set_default_toolchain.cc
diff --git a/tools/gn/function_set_default_toolchain.cc b/tools/gn/function_set_default_toolchain.cc
index 1eb5ee09b818ffa26334a9cb61b56d7e0dbddecb..08b9d12251b14fa98b36e190f9b2edb3fd879ce4 100644
--- a/tools/gn/function_set_default_toolchain.cc
+++ b/tools/gn/function_set_default_toolchain.cc
@@ -67,7 +67,8 @@ Value RunSetDefaultToolchain(Scope* scope,
   if (!EnsureSingleStringArg(function, args, err))
     return Value();
   Label toolchain_label(
-      Label::Resolve(current_dir, default_toolchain, args[0], err));
+      Label::Resolve(current_dir, default_toolchain, args[0],
+                     scope->settings()->build_settings()->root_path(), err));
   if (toolchain_label.is_null())
     return Value();