debug_stub: Avoid unused const warning on non-x86 builds
debug_stub: Avoid unused const warning on non-x86 builds

The current Clang used to build in Chromium warns about an unused
namespace-level const variable.  This variable is unused in
ARM (or any non-x86) builds.

BUG= none
TEST= trybots
R=mseaborn@chromium.org, bradnelson@chromium.org, gdeepti@chromium.org

Review URL: https://codereview.chromium.org/1354263002 .

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client