Subzero: Change szbuild.py to prefer Subzero by default.

Subzero translation is stable enough that szbuild.py should prefer Subzero-translated symbols by default.  The exception is that if you explicitly use --include, the intuitive interpretation is that you only want Subzero to include those symbols (minus any given with --exclude).

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/605283002