Use LLVM args to handle -malign-double option

We previously used CPU features to pass the align-double option from
the driver to the frontend, but these passed to backend causing
spurious warnings. The implementation now uses LLVM args instead
which doesn't have the same side-effect.

BUG=none
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/1323073010 .