Build Subzero when using the cmake build.
Build Subzero when using the cmake build.

This requires subzero and llvm changes to take effect, and is
otherwise harmless.

BUG= none
R=jfb@chromium.org, mtrofin@chromium.org

Review URL: https://codereview.chromium.org/992493003

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