normally this warning is disabled but for nacl builds that
use clang its not.  this CL makes the option universally disabled
for all clang based builds.

BUG=libyuv:581
TESTED=native_client/build/gyp_nacl all.gyp -Dgtest_target_type=executable -Dmsan=0 & ninja -C out/Debug libyuv
R=kjellander@chromium.org

Review URL: https://codereview.chromium.org/1865183002 .