Build a pure-Subzero binary when neither --include nor --exclude is specified. A pure-Subzero binary is built without flags: -externalize -ffunction-sections -fdata-sections which is good because that configuration is closer to what real usage will be, and will get more testing during development. BUG= none R=kschimpf@google.com Review URL: https://codereview.chromium.org/693393003